We've just uploaded a updated of the Eclipse plugin on http://plantuml.sourceforge.net/updatesitejuno/
But we had to keep the same version number (V8051), so upgrading from your already-installed Eclipse would not be that easy.
First, you should test on a fresh install of Eclipse (in some new folder c:\neweclipse)
Run this new eclipse, install the plugin just to check that it works with AsciiMath (we have tested it on our side).
If it works, we can try to upgrade your existing eclipse: try to force a reinstall of the plugin to see if it works there.
Keep us informed!