Once your Tascam mixer is connected to your computer and is on, open your DAW > options > preferences > MIDI > Input: select your midi device > Output > Select your out ports to wherever you'd like ( or leave blank ).
Then look through the DAW and find an option for Midi Control surfaces (Check the programs manual if you don't know what this looks like). Once you find it, enable it. It's possible there is already a map listed in the options that will make the mixer work automatically.
Drag a song into your project (Any Song) , open the DAW virtual mixer and and take a close look at your settings, you should be looking for a button that says "read" and another that says "write". If you click "write" and then press play on the DAW, if the tascam is connected and functioning, you will begin to see midi control data being printed as you adjust the potentiometers while the song plays.
Once you're satisfied that the DAW is writing the midi data to the track, click the "Read" button and deselect the "Write" button, now press play. If everything is working as it should than the midi data you printed to the audio track will begin controlling the physical potentiometers on your Tascam, like magic.