When on XM360 did you perform the “Rescan All” option? After that you should be able to see all DLC, XBLA, GOD, TUs, etc… You may need to look for a “Show DLC” button" or use RB to select the category. Try that and let me know what’s up if you haven’t solved it yet yourself.
About the TU. I still encourage you to check that you indeed have the correct 18. There are at least 5 different media IDs for 18 out there.
EDIT I haven’t fired up either of my RGH’s in months now. If my suggestions don’t work I’ll break one out and see what I can figure out. I will add that if you have both and internal hard drive and external hard drive make sure you are looking at the correct one when in XM360 or any other app. And make sure the files are in the correct locations.