All the PCR tuners contain a USB>Serial adapter, so yes, guessing that what you're doing is using a USB>Serial connected to an XMD, it should work for you.
What kind of USB>Serial adapter are you using, and can you elaborate a little on what you mean by: "it will now allow me to check the box to enable it"? The reason I ask is because I don't think there's any logic to disable the checkbox. There are a handful of reasons why you might be having this issue, so the more detail you give about your setup the greater the likelyhood I'll be able to help. Checking your debug.log file and posting your settings.ini file would be a good idea too.