You are probably working with typed datasets. If that is the case, the table adapters for your datasets will be generated by Visual Studio. The type of the table adapter is not known in your code, resulting in the error.
That means that you did not define the typed dataset (and the tableadapter) in your assembly or you did not properly reference the assembly where it comes from.
Here is some info on strongly typed datasets
] on MSDN to help you get more familiar with them.