What's the name of the file you're trying to open?
You don't run install on the actual plugins, you just place them in your VST folder (after you've unzipped them). The VST folder is usually C:\Program Files\VST\. If you don't have a VST folder, you can create one and go into Mixcraft's miscellaneous (or effects in v3.0) preferences to make sure it's looking in the correct folder for the plugins.
Note: I'm going to move this thead to the Mixcraft forum, so look for it there.