The Secret of Monkey Island
She can be encountered on Mêlée Island in the International House of Mojo. Though you do have to enter her house to get a rubber chicken, you don't actually need to meet her.
Monkey Island 2: LeChuck's Revenge
Guybrush meets with her in Scabb Island's swamp. There she gives instructions on how to create a voodoo doll of Largo LaGrande (namely, getting a piece of cloth worn from him, some of his hair, some of his bodily fluids and something from a dead relative). She then creates a doll for Guybrush so he can use it on Largo to get rid of him.
Later on, Guybrush has to ask her to create an Ash 2 Life potion so he can resurrect Rapp Scallion to get his piece of the Big Whoop map.
Finally, Guybrush is asked by Wally the cartographer to retrieve a love potion from the Voodoo Lady.
The Curse of Monkey Island
She houses on Plunder Island. There, she gives Guybrush clues on how to get rid of the curse that turned his wife into stone.
Escape from Monkey Island
She goes back to her initial House of Mojo on Mêlée Island. There, she helps Guybrush sorting all the items in Grandpa Marley's chest, items which will be used to find the Ultimate Insult. Though she can find three of the four items necessary, she can't find the fourth and urges Guybrush to find the fourth one, somewhere on Mêlée Island.
Tales of Monkey Island Chapter 1: Launch of the Screaming Narwhal
It is revealed that as he went for LeChuck to save his wife, Guybrush used a mystical sword called the Cutlass of Kaflu that was provided by the Voodoo Lady. However, as he used the Cutlass with substitute voodoo root beer, the Cutlass instead of destroying LeChuck turned him into a human.
When he ended up on Floatsam Island, he eventually meets the Voodoo Lady, masquerading as an informant for the local newspaper under the name of Deep Gut. Upon seeing Guybrush, the Voodoo Lady asks for Guybrush to give her back the Cutlass of Kaflu, but the answers given by Guybrush threw her in a rage. The Voodoo Lady explains that as he turned LeChuck into a human, the voodoo magic contained within him spread in the entire Carribean and nicknames it the Pox of LeChuck. She urges Guybrush to find La Esponja Grande, a voodoo artifact capable of sucking the Pox out of the air. She also gives Guybrush a locket that he has to give to Coronado De Cava, her former boyfriend, and asks him to find the explorer as he went for La Esponja Grande many years ago.
Tales of Monkey Island Chapter 2: The Siege of Spinner Cay
Aside from seeing her in the opening scenes, narrating, the Voodoo Lady is seen in a cameo at the encampement of Coronado De Cava. De Cava holds a photo of him and a much younger version of the Voodoo Lady, which Guybrush can make a comment on.
Tales of Monkey Island Chapter 3: Lair of the Leviathan
Aside from the opening narrating scenes, the Voodoo Lady appears as she is communicating with Guybrush using the locket given to De Cava. When De Cava understands that the Voodoo Lady is giving Guybrush some quests, he gets jealous of Guybrush as he thinks she is cheating him with Guybrush. It also appears that giving quests was at the heart of the De Cava-Voodoo Lady relationship (even though judging from Chapter 1, they seemed to have sex).
Later on, Guybrush uses the locket to take control of the Voodoo Lady in order to get a book from the Marquis De Singe which would let him learn the language of the manatee.
Tales of Monkey Island Chapter 4: The Trial and Execution of Guybrush Threepwood
LeChuck (while still a human) interrupted the trial before the judge can sentence Guybrush. There, he explained that Guybrush is innocent because the Pox is his but also claims that the Voodoo Lady is responsible of the whole thing. According to a journal owned by the Voodoo Lady, she has indeed been manipulating both Guybrush and LeChuck for all those years. Guybrush is then released from his prison and both LeChuck and the Voodoo Lady are imprisoned. Despite being shocked at this sudden revelation, Guybrush decides to trust the Voodoo Lady in order to bring la Esponja Grande to maturity.
Tales of Monkey Island Chapter 5: Rise of the Pirate God
The Voodoo Lady appears right during the closing moments of the game, as she cut a deal with Morgan LeFlay. In exchange for LeChuck's voodoo essence, The Voodoo Lady will bring Morgan back to life (or unlife, rather, as LeFlay states she will be a "Ghost Pirate Hunter", similar to how LeChuck escaped the world of the dead as a Ghost Pirate). As she leans over LeChuck's essence, she starts to chuckle. The implications of that scene are still unknown, although it is safe to say that LeChuck was right about her.