When you find a recipe you would like to cook (from my bookshelf/recipes), click on the bookmark with a plus sign. On the list of recipes page, it is on the bottom right of the recipe (the same line as the stars & the notes field). Then click on the "I want to cook this" box. This is also where you find the "I have cooked this" box, or any personal bookmarks you have made for recipes. (A recipe must be on your bookshelf before you can add a bookmark for it.)
If you are on the page for an individual recipe (where you see the recipe title & the book title at the top of the page, and then the list of ingredients), then look for "My bookmarks" on the right side of the page, under the stars for EYB member ratings. Click on the plus sign next to "My bookmarks".
Bookmarking a recipe with "I want to cook this" will give it a "red hat" next to the recipe. Bookmarking a recipe with "I have cooked this" will have have a "green hat"; this option also lets you specify a month & year that you've made the recipe, and has space for your notes.
You can see all the recipes you have bookmarked by going to My bookshelf/My recipes, and then look on the right columm for dropdown menu My bookmarks/Show only recipes. Choose the appropriate bookmark. You can also limit a recipe search to things you have bookmarked (i.e., chicken recipes with the "I want to cook this" bookmark).
Hope this helps!