OK, I've reproduced the problem and tracked down the cause.
It looks like the dependency list in the packaged feature/plugin is incorrect and points to an older version of one of the underlying supporting plugins.
I'll be packaging up a fixed version of the BrightScript plugin soon.
If you can't wait for the update, there is an easy workaround:
Use the software install dialog, and install the DLTK 2.0 Core Frameworks plugin from the DLTK update site (
http://download.eclipse.org/technology/dltk/updates/)... then restart Eclipse, and you should be good to go.
Cheers