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.
what is eclipse anyways?
Well, I tried installing Eclipse on my mac the other day, and it turns out both the latest version and the earlier (3.5?) version don't work on Tiger. I don't expect to be able to upgrade my mac any time soon so I can't use it. I'm using Jedit. Wish there was a plugin for that.