Before attempting any of the following, be sure to backup all personal files and documents!
First, you need an upgrade CD to WM5 for your Axim x50v. I assume that the "OS Disk" you mentioned is the upgrade CD. Check if it says x50(v) on the disk itself.
Second, once you have upgraded, you now have WM5 A01 English. Now you need to upgrade to the WM5 A02. Use this link:
here
Third, you can now upgrade to WM6.1 for the x50v. Doing so will void your warranty with Dell (I doubt you have any). If you brick your device, Dell will not repair it. But we will!
You have a few options for WM6.1 ROMs.
1: Maglite's A02 RC2:
here
2: Lenny's A09:
here
3. Makuu's VOIP A09:
here.
I personally recommend Lenny's A09 Test 2, that seems to be the fastest. But it comes with other programs you may not need. If you just want a barebone ROM, use Maglite's.
Once extracted, you will find a .nb0 and .crc file. If you do not know how to flash, continue to reply to this post and I will help you.