Will this also work for MOSS 2007?
It is not mentioned, that it will not work for MOSS 2007 or that it even depends on SharePoint at all, but I get the error "Cannot find custom tool 'MaventionMinifiedCssFileGenerator' on this system."
Although I haven't tested it, it should theoretically work. The bug that you are mentioning, is something I have stumbled upon myself at some point. I thought that it had something to do with me installing VS11, but apparantly there is more to it. I'll see if I can find out anything more about it and will let you know if I find anything.
Have you downloaded the latest version? There were some issues previously but those all should be fixed in the latest version. Have you changed anything about your Visual Studio setup recently by any chance?
I am also getting the error with Visual Studio saying it cannot find the tool. I came across it when I noticed that edits to the debug file weren't updated in the minified version automatically. I right-clicked on the css file and clicked Run Custom Tool. That was when VS says it cannot find your custom tool.
I do have the latest version and recently tried re-installing it.