Instead of using the Dell add on, you could try one of the other two methods I can think of;
1. If all you want is the asset tag info, you could use a software distribution to run ASSET.COM on your machines and dump the output (which is the Asset tag in the BIOS) to a NOIDMIF file that will be inventoried by SMS2.
2. You could also try upgrading all of your clients to WMI 1.5 (9x machines install a lower version of WMI on installation of the SMS 2 client), which will send the DMI (cross-manufacturer standard)information from client to the server. This, however is similar to using the DELL MOF extensions, so you will still need to fix the server MOF like WhiteWiz says to do above.