Quite Simply, you do not have Market really installed on your Tablet. There are instructions available on this and other forums about how to really install Market. What some vendors do is they install the Market app, but do not have permission to use Market on your PC. Market depends upon three things. You are likely missing #3
1. Installed Market App
2. Valid Gmail Account which is registered with Market
3. Valid Unique Market identifier on your tablet. This is done in a couple of ways. Most common is using the Android SDK and moving that unique identifier created to your Tablet.
Search, find it, follow the instructions and you likely can then really have access to Market.