Once you've picked a synth (the Chrome browser ones are really quick to try out, hint, hint!) plug in your Trellis M4 and get playing! It's very likely the synth will recognize it immediately, but if not, check the preferences and choose the "Adafruit Trellis M4" MIDI device.
Here's an example using Helm.
The note on/off functions will work immediately, as should the pitch bend if it's supported in your software. Try holding some notes and then tip the controller left and right to bend down and up.
The forward and backward tilt will send CC 1 messages, which is usually interpreted as "mod wheel". This can be mapped by default to things like tremelo, but you may want to change it. In most software synths it's a matter of checking the help docs and reassigning it.
For example, here's how to re-assign a MIDI mapping for the x-axis tilt in Helm so that it will control Filter Cutoff Frequency:
And, here it is in action!
Enjoy playing with your new Trellis M4 Expressive MIDI Controller! If you want to further customize things, check out the page on coding it in Arduino.