Today I had a different issue when in I started to work on a BizTalk Sample created by a colleague.
It has a source and destination schema, which are to be mapped.
There are no build errors and every thing looked fine.
The culprit which led to this is the Build Action property of the schema.
This property will be shown when you select the schema in the Solution Explorer.
If this property is set to None, it will not be included in the Build and hence will not be able to use that in BizTalk Tools.
I made the Build Action to BTSCompile and it showed up in the BizTalk Type Picker of the BizTalk Map.
The reason being for this is that the schema was not created using BizTalk Schema Editor rather using an external tool. Hence when this schema was imported to BizTalk Project, its Build Action property was set to None.
So, care should be taken for the schemas that are created outside BizTalk Schema Editor.
Hope it helps.