… iMovie 11, would not play former versions. …
and as addendum zo Bengt's excellent advice:
iMovie is no 'player'.
iMovie projects, from any version, are meant to be shared/broadcasted/exported (diff. words in diff. version, meaning all the same).
Such exports, from any version of iMovie were, are, and for sure will be playable - with Quicktime Player.
So, meant as generic advice:
Don't 'hide' your Projects in iMovie!
Export them, make a mov (or m4v, or mp4, or whatever) of it - done.