Dave,

Each of those files you downloaded contains two MIBs. The second one in each case is nothing but TRAP-TYPE definitions. The mibloader checks them for syntax but does not compile them. He leaves them for mib2trap. What I would do is this.
(1) Comment out the iso definition at the top of the first MIB in each file.
(2) run mib2trap on each file. That's probably why they come concatenated that way any how -- for mib2trap.
(3) when mib2trap is done, edit the file and remove the second MIB from the file altogether. Make a back-up first in case you run into a problem.
(4) Then use xnmloadmib on what's left.

If you still have problems, let us know here, but if it gets too involved, you'll have to call Support
HTH

James Shanks
Level 3 Support for Tivoli NetView for UNIX and Windows
Tivoli Software / IBM Software Group