loss tangent cst
For your problem, you need to write epsilon and mu in terms of tangent delta electric and magnetic. (For dielectric and magnetic losses.) You need to also enter conductivity losses, if present. Just look at the freq dependent formulas for complex eps and mu, in an advanced EM book...
Off topic:
When I was working with metamaterials in the past, I tried to run a hypothetical problem in CST, which would include negative epsilon/mu. As it turned out, CST did not allow eps<1 and mu<1. Then I switched to HFSS.
This was in CST 5.0, i guess. Is it still the same for the newest CST version? Just wondered...