I remember seeing a youtube video about it but I have forgotten. Thanks for your help <3

Depends on what you want to modulate? o.O

I have a voice sample in a dr rex and I want to attach a malstrom to it and use the malstrom's B modifier on the dr rex.

I was trying to do the same thing from NNXT to Thor last night, but I couldn't figure it out.

@djenept:
What do you exactly want to do with the B mod of malstrom? It's just an LFO and you can link the corresponding CV-Out to any CV-In of any other device. (like "OSC Pitch" of the Dr.Rex).
Or did you mean the FILTER B? Then you have to link the audio output of Dr.Rex into the audio inputs of malstrom (they are on the right side of malstrom)

I want to link the malstrom's MOD B to the Dr Rex so I can give it a wobble kind of sound in terms of volume wave in and out.

OK, then you can do it alike I said above. Link the MOD B CV out to the "Level"-CV in of Dr.Rex.
But you can just use Dr.Rex's own LFO to do this, too. Just connect the LFO CV out the the level cv in.