Runeforge Gossip have a good short article on how to use Rawr as a DPS DK. It can be a fairly confusing tool at first with all its options even though on the surface it appears simple. There are also optimisation and gear-search features which are worth understanding.
The DK tank module within Rawr is still being developed, but one thing I do like about it is their model for comparing weapons lets you weight threat or survival. It seems fairly accurate, and has a lot of weapons loaded in the database. So don’t focus on the the TPS estimate the tank module gives you, but do use it for gear selection.