(You'd want to add all information here, which is where support happens). That dialog doesn't look dead, FWIW. You should be able to drag it by clicking either at the bottom or anywhere in the section with the selected item (that isn't actually the selected item).
And either return or the checkmark should insert the citation.
The checkmark is shown in your screenshot — the top right of the window, next to the X. Enter should work as long as your cursor is in the input field (next to "Davis, 2009" in your screenshot).
The checkmark in your screenshot is not grayed out. It would be grayed out before you add an item to the citation, but you have an item in your screenshot — "Davis, 2009".
Can you provide a Debug ID for 1) clicking Add/Edit Citation, 2) adding an item to the citation, and 3) pressing Enter in the search field? It's possible you're hitting some internal error here that's preventing the dialog from closing, even though the button is active.
Upgrade Storage
https://s3.amazonaws.com/zotero.org/images/forums/u143964/x58fawwxb52v1r2gx61z.jpg
That dialog doesn't look dead, FWIW. You should be able to drag it by clicking either at the bottom or anywhere in the section with the selected item (that isn't actually the selected item).
And either return or the checkmark should insert the citation.
This is grayed out:
https://s3.amazonaws.com/zotero.org/images/forums/u6/sc2en0qoy73zwmvhs8lv.png
This is not grayed out:
https://s3.amazonaws.com/zotero.org/images/forums/u6/3w2gpowzo1lkayq7fvvy.png
And this is your screenshot:
https://s3.amazonaws.com/zotero.org/images/forums/u6/4qal4jjpks7suec93lhk.jpg
Yours is lower resolution, but you can see that it matches the second image.
Can you provide a Debug ID for 1) clicking Add/Edit Citation, 2) adding an item to the citation, and 3) pressing Enter in the search field? It's possible you're hitting some internal error here that's preventing the dialog from closing, even though the button is active.