There are the 2 main ways to do it.
From the page of the list you want to add to:
- Click on the button labelled "Edit your list" at the top of the right-hand sidebar.
- Scroll to the bottom of the list.
- Type into the search whatever you want to add.
- Click on the search result you want once it appears.
From the wiki page of the thing you want to add:
- Click on the button labelled "Add to list" on the right at the top of the page.
- A pop up box will appear showing all of your lists. Click on the list you want to add the item to; it will be appended to the end of that list.