Bob,
MyEclipse 5.0 is fully compatible with Callisto so it’s odd that you had an issue. Could you give our installation instructions a quick read and see if there is anything in there that looks like you did differently?
However, I’d guess that the issue was likely that the Eclipse plugin cache got stale as it sometimes does not cleanly rebuild after updates. To rectify the problem you likely could have simply needed to restart Eclipse with the -clean flag one time: eclipse.exe -clean
That should have resolved the problem without the “do over”, so it’s a good tip to have around if things go pear-shaped next time.