just to be clear, what is it that your wanting to add the effects to; individual
instruments in your mixes, or over the whole mix in general.
And also, what
kind of effects. Is it insert effects like compression or limiting the mix or whatever, or
send/return effects like Reverb, delay etc etc.
By the sounds of it its the
whole mix so i'll describe that, which is also the easier one to do.
If your
just wanting to apply some insert effects to the whole mix, for example to compress or
limit a little, or put a bit of mastering EQ on or whatever, its easy, simply plug the
computers output the effects unit input, and the output of the effects unit into whatever
input your using in the desk.
For send/return type effects or to put the
effects on individual instruments, it's more complicated. The easiest way to do all this
would be to invest in a patchbay. Because you only have a single output in your sound card
you'll have to change the wiring slightly depending on whether your using send/return
effects or insert effects. Obviously you can do this by climbing behind and re-aranging
the wires in the equipment but after the 2nd or 3rd time it'll be a pain in the a***.

If you can give me a few more details i'll be happy to help if i can.
kev