Select value from formula completion drop down list, by keyboard
Please find below link from Allen Wyatt's website.
http://excelribbon.tips.net/T010181_Selecting_a_Suggestion_with_the_Keyboard.html
I was also looking for this also before but didn't bother to find out until I've read this post.
The answer is "TAB" key after you have selected the function/formula from the list by arrow down/up key.
Try TAB instead... ;)