To use an external effect device with DP and the 2408, I'll use input 3 and output 3 as an example. You can use any i/o, and a stereo pair (like 3-4) if you want to, or the equipment warrants using stereo.

1. Connect the FX unit output to 2408 input 3
2. Connect the FX unit input to 2408 output 3
3. In DP's Mixing Board window, assign a Send on a track to "analog 3" output
* (see p598 of the DP3 Operation Guide for info on sends).
4. Raise the level of that send.
5. Do the same for other tracks you'd like to have use this effect.
6. Create and Aux track. Set it's input to Analog 3, and it outupts to your main outs (usually analog 1-2).