Hi,
The user selector there is kind of tricky we need to improve it but it does work if you can get it to save the user id. What I do is type the first few letters until I see the user name, then press the arrow down key to get into the dropdown and arrow down to the correct user, then tab out. You should see a user id there before you save.
We will look into making that easier to use.
Hope that helps,
Joe