assign all of your drum tracks to play back outs 1 & 2 of the sound card. send them into the mixer. if you are micing a guitar amp. plug your mic into the mixer, and send the direct out to channel 1 of the sound card to be recorded. You can then monitor the drums and guitar from the control room out of the mixer. If your guitar is being recorded in the same room where you are monitoring, you will probably want to use headphones instead.
EDIT:
You said your mixer has Direct Outs. I dont know if it does but ill bet those are inserts. If so, and you use a TS plug (mono) to go to your sound card, you probably are not getting sound in your monitors. Inserts take TRS plugs (stereo). If you are using TS, only insert the plug about half way, to the first click, this way, the sound gets to your sound card and you are still able to monitor it.
hope this helps...