I never had much luck with the MIDI OX program. I use Bome's Send SX, and it seems to work whenever I need it. Download and install the 1.22 version from that link (you could do 1.3, but it's still officially a beta release). Open up the program, select your adapter from the "MIDI out" pulldown menu, open up the M13's sysex file, and once you have the M13 in update mode per the instructions, you just hit play and the file will send.

On the hardware side, it matters what USB to MIDI adapter you're using. The M-Audio Uno is what Line 6 recommends. Other quality ones will work, but avoid the ones you get on eBay or Amazon for $5.00. They seem to have constant problems. After that you'd take the adapter's MIDI out into the M13's MIDI in. You don't need to worry about having a cable from the M13's out to the adapter's in. (The M-Audio Uno actually labels its cables "to MIDI in" and "to MIDI out". The labels are correct, but it's a bit confusing because the one labeled "to MIDI in" is actually its output and vice versa.)