To edit your biography you first want to make sure you are on your profile page. This is a different page than the one showing your tree and G2G feed and such. If you are anywhere on wikitree you can get to your profile page by going to "my wikitree" on the top right of the page and hover over it with the cursor to see the drop-down-list. On that list is the option "profile"... click that. Once on your profile page you can select "edit" from the drop-down box. Once there you should have no trouble adding to your profile. When you are done make sure to save the changes by selecting the large green button on the bottom of the page.
Have a great day.