How do you use NSLOOKUP with MX records?

At the command prompt type NSLOOKUP (and press Enter).
This will give you a > prompt.
Type set type=MX (and press Enter).
Type simpledns.plus (and press Enter).
You should now get the MX records for "simpledns.plus".

An easier alternative (which also works on Win9x) would be to use the DNS Look Up tool in Simple DNS Plus.