The easiest way is to email him at:
You can send him a letter using dearelder.com This is a great way to get a letter to Mitch in Cambodia (and its free). They will print it and have it sent through the Church's pouch system. Letters sent by midnight Sunday will be printed Monday and sent that week. Select "Cambodia Phnom Penh" in the drop down.
Do it the old fashion way and put a stamp on it. You can also send packages to this address:
Elder Mitch Neuberger
House 2B, Street 222, Off Norodom Blvd
PO Box 165