There are some substantial examples in the Castalia manual that may guide you when creating WSN scenarios. There you will see how to define sensor nodes, their protocols and basic configurations, along with configurations of the simulation process.
In order to test T-MAC operation, specify it as the MAC protocol to be considered. It can be done as a simple command in a single line. Once again, check the manual.
You can use T-MAC with default parameters or configure parameters as sleeping time and synchronization.
Hope I have helped.
Regards,
Daniel