OK, not to insult your inteligence, but I'll start from the begining:
1. You have to pull the little plug out of the output optical jack on the back of the ua-5. Save it somewhere. When you take the cable out, put the plug back in.
2. Toslink cables are not square, They have a top and bottom to them. If you do not have it lined up right it will not go in. The top corners are angled. bottom is 90*
3. If that does not fix it for ya, you might need to pull the little rubber protective ends off your cable (if it is brand new then this is the problem)
JAson