I've had good success with panels designed for FS5.1, but have found
that all those new panels specifically designed for FS98 do NOT work
with FS5.1. So, first off, make sure that the panel you're trying is
made for FS5.1, not the later sim.
Here's what I do (though I suspect you've already done it):
Suppose the new panel is called 747-400.zip. Unzip things
and put all the various files .gau, etc into fltsim5\panels. One of
them is called 747-400_s.pnl or some such (I think of it as a
"panel assignment" file). Copy this one over jet4_s.pnl (which assigns
the panel to 4-engine jets) and I'm done. Hope this helps.
-- Dennis S, FSEngEd@AOL.COM