Chrome is actually lying to you when it gives that notification. If you open the extensions page and drag the .crx file into it, Chrome should install the extension, even though it will try to tell you otherwise. Step-by-step instructions and more information can be found here, if you need it.

We’ll get it on both Firefox’s and Chrome’s addon stores once we fix a bunch more stuff (old XKit really wasn’t that stable), but until then this should work. If you try the above and it’s still not working, shoot us another ask.