I'm at a loss to work out what the problem is.
To trouble shoot, first work out if the levels going into and coming out of the mackie all ok. Being a mixer, it should be able to handle all types of inputs from mic to line level. If you plug headphones into the mackie and play the guitar, can you hear the signal nice & loud?
If not, then that's the first thing to try to fix.
Are you DI-ing the guitar or micing it?
If using a mic, try the old "one two" test and see where the recording levels are in your software (which software are you using - maybe there's someone on the forum who can help you with that side of things?)
If using a condenser mic, make sure you have sent phantom power to it from the mixer.
if DI-ing, get a fresh battery, turn the guitar volume all the way up (if there is a volume adjust) and try again.
If all is in order and the signal is nice and loud going into and coming out of the mixer, then plug the outputs into the tascam and check the levels in the software. Turn the gain up on the channel inputs of the tascam until the signal is at a good average level in the software.
Also - consult the tascam manual which may list connection tips to follow.
Best of luck!!