Get one of these from Stew-Mac.
STEWMAC.COM : Fender-style Bullet Truss Rod Nut
Then CAREFULLY back the existing one out (that's probably going to be the most difficult part, and helping you via the 'net isn't going to be easy), put a little LIGHT lube on the threads. I use bicycle grease because it won't get into the wood and I always have bike grease (NOT chain lube, but the grease for packing bearings) around the house.
Always use the correct sized allen key, make sure it's firmly seated and aligned before you start twisting things, and ALWAYS pre-stress the neck before you tighten the nut.
John