I would start by getting Putty on a Windows PC and a USB to DB9 adapter for your computer and connecting it RIGHT to a computer to ensure you are sending the proper commands. Plenty of YouTube videos on how to use Putty for RS232 control out there.

You should use a straight through M/F DB9 cable (serial cable) from the PC to the Xantech unit. Open Putty, then start trying commands.

It does not look like you need to send a carriage return for the command to take, so that shouldn't be necessary. Just send a command and look for a response from the unit to your computer.

For example:

?SI+

You should get a bunch of information back about the unit.

Make sure dip switches are in the proper position as dictated by the manual.
I would leave all four switches in the 'off' position.

If you want, you could pick up a IR remote on eBay, but they aren't really cheap...
[Link: ebay.com]

Overall, while this is a interesting product, I would be more inclined to use a Extron Crosspoint RGBHV switcher w/audio as it can act similarly and has the front buttons along with very easy RS232 control. But, it doesn't extend over cat-5e if you need that.

Manual I looked at was here:
[Link: corebrands-resources.s3.amazonaws.com]