First things first: Understand the basic concepts... TouchDAW is two programs in one and uses a dedicated MIDI connection for each of them:
To make full simultaneous use of both parts you need to set up two MIDI connections and take care of properly routing them inside your DAW. Many FAQs and half-informed Play Store comments directly result from people assuming they know how a MIDI controller will work, while clearly not understanding these fundamentals.
MIDI connection setup
Making network connections w/ TouchDAW's default settings ( WiFi / RTP / Local Session ) and:
Welcome to TouchDAW's 'horrible setup', originally designed by a large company from California in 2005. Try mnet if you do not like this.
Only steps 8 and 9 need to repeated on subsequent runs.
Once you are familiar with the rtpMIDI / OS X control panels you can also set TouchDAW to autoconnect to the sessions you have created.
Some may prefer a video (YouTube link):
Not using an old Cubase version? The important thing here is to understand the principles. Setting up the MIDI driver is the same regardless of the DAW you are going to use it with in the end. Setting up a Mackie controller is also pretty much the same with other DAWs, except that the program's windows may look a bit different. The DAW setup manual page contains more detailed instructions for all supported DAWs incl. screenshots and links to available video resources. It is also generally a good idea to make yourself familiar with what your DAW's manual has to say in terms of Mackie Control. Hints on where to find the various vendors' remote control docs are also given on said DAW setup page.
Additionally you can look at video tutorials for the various iOS control surface emulators (AC7, DAW Remote, Eyosido etc.). They use the exact same functionality where they speak of "Core MIDI" or "Network MIDI"
It has not escaped my attention that the whole session / participant thing appears to be everything from "simple" via "not overly easy" and "fiddly" to "absolutely impossible to set up" to people and I agree: This is not the most intuitive way of plugging in a MIDI cable. However, if you look back for a second, you will realize that you did not have to do anything in the app itself and all the complexity results from Apple's 15 years old and well established MIDI network concept which rtpMIDI replicates 1:1. TouchDAW is about working with standards and this is the most common (free) standard available. Once you get used to it, you will probably realize that it was only as complicated as you might have made it.
Find information on other connection methods in the manual.
See FAQs / Troubleshooting should you run into problems.