That's one of the basic distinctions between pop-up's and drop downs. You can't type text into a pop-up menu. The only work around I can think of requires a script.
You'd select the item from the pop-up and either click a button or use an OnObjectModify script trigger to run a script:
Set variable[$Name; YourTable::YourPopUp]
enter find mode
Set Field [Yourtable::YourPopup; "==" & $Name]