Hmmm, what's your MIDI controler? I mean, was it just plain controler (no sound), or it has a sound of it's own? Do you want to record MIDI data or audio?
I don't use Acid, so please bear with me. Usualy, decent sequencer will alow you to call soft synth/sampler as tone generator. You then use the synth/sampler as MIDI tone generator to make sound (audio). All you need to do is to insert the softsynth, load the sample sound/preset, assign MIDI track input to your MIDI controler port, and output to the softsynth. Then you record MIDI data into MIDI track as usual. The data being recorded is MIDI data of your performance, which will trig softsynth/sampler to make sound. Once you have recorded the MIDI data, you can edit it later, or assign different sound to the track. Please refer to Acid manual on how to set your track to record MIDI, and how to assign soft synth/sampler as tone generator.