USA now uses Game Officials for all our referee assigning and score reporting.  
Group Code: 1290
Access Code: 1986

Go to https://www.gameofficials.net to become a MUSA referee. 

Don’t forget you must also send your completed W9 form to the Treasurer at 
MUSA Treasurer
PO Box 517
Richmond, Kentucky 40475

Download the W9 tax form here: musa_w9.pdf

Referee cheat sheet
Advice for new refs
Pre-game conference
Bench control
Foul challenges
8 Keys to communication

Assistant referee documents
FIFA: Assistant referee
Linesmanship
Offside duties
AR Involvement

Laws of the game an other FIFA publications
FIFA Publications