Have two Macbook Pros, can't update iMovie on either
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?