Here's the trick to fix this problem. Open the style in EndNote X. Pretend you want to edit it (but all you really are going to do is rename it). Do "Save As..." so that you can save it with another name (such as [oldstylename]_new. Once you've given it a new name, then EndNote X2 will open it, since it can read styles from EndNote X, just not from really old versions of EndNote.
Hope that helps someone out there! AG