I received RNA seq files (.fq) and I was told that the sequencing facility used Nugen (Ovation RNA-Seq Systems 1–16) kit for library prep. The seq was a directional and paired end. 

Can anybody tell me where can I find adapter sequence. and how can I trim these adapters using Trimmomatic?

This isn't the right forum for these questions. You should contact NuGen if you want information about their product, and you should look at the help for Trimmomatic if you want to learn how to use their tool.

Or you could note that modern aligners are quite capable of soft-clipping the ends of reads, so for most applications it's probably not critical to bother with adaptor trimming anyway.
 

Can STAR do that?