Have two Macbook Pros, can't update iMovie on either I have an old MacBook Pro running iMovie 9.0, and a newer Macbook Pro with version 10.1.1. I am trying to get them to match. 

When I try to update the old one to use it for my movies by going to the App Store, there is no update listed in current updates.  When I select iMovie as if I'm going to buy it it says "update" in the box, but the asterisk-like wheel just circles forever. I have tried trashing the application, signing out and back in as suggested, but nothing changes.
Mac Apps have limitations as to which operating systems they can be installed on. You mentioned a newer MacBook Pro on 10.1.1 (did you mean 10.11? If not, keep reading). You need to first update your operating system, and then update iMovie. If you are unable to update the computer because it is too old and the new OS doesn't support it, you can get your copy of iMovie from Purchases in the Mac App Store, but it might not be updated. Note that downloading from Purchases is not an update, but rather a replacement to the existing copy that shipped with your computer.

This question has been asked here, so read there too: Why can&#39;t I update iMovie?