Welcome to the forum.
The error you are getting is real. TypeMock can not mock types from mscorlib.
The work around around this problem is to wrap the mscorlib type in your own class and mock your class.
As for your second question:
MockManager.Mock will mock the next instance of the type
(in other words next time you use 'new')
MockManager.MockObject creates new object and mock it.
It is really usefull when you want to mock an interface or abstract class.
See more details here
Hope it is clear.
If you have more questions We'll be happy to help.