You've left out some information: computer MIDI interface + platform. If you're using a standard sound card on a Wintel machine, you'll need a gameport/MIDI adapter cable, which will give you one each MIDI in and out to your computer. I suspect Mac is similar, but I have no experience with that.
Since I presume your Roland is your controller, hook its MIDI-out to the adapter cable's MIDI-in. The MIDI-out of the adapter then goes to the MIDI-in of your sampler. If you want to use both the Roland and the sampler as modules, then you'll need to use MIDI-thru as the output of one to go to the other's MIDI-in.
Getting multiple MIDI-outs from your computer (allowing patching directly to each outboard module/synth) requires a MIDI patch panel - an additional piece of hardware. The standard soundcard setup will require you to filter the MIDI input of each device so that both don't play everything. Should be fairly easy to set up - consult the hardware's documentation on how to select channels for input on each. Of course, this isn't necessary if you're only using the sampler as a MIDI module.
I haven't mentioned anything about using your mixer for "real sound" sources - if you need help with that, more detail about what you want to mix with what and where it's all going are required.
Hope this helps! I'm not emailing directly, 'cause a forum benefits everybody. Hope you're checking here!
"The problem with the world is that the ignorant are cock-sure, whereas the intelligent are full of doubt." - Bertrand Russell