Basically you need two things. One is a ping program (app) for your iPod touch, so search the App Store for that (should be free). Then you need to figure out the IP address of the Windows computer. Running "ipconfig" from a console should do it, or you can click around the Network connection and find the dynamically assigned address. Anyway, the address of your Windows machine is the one you want to ping.
That said, MIDI Designer isn't part of the problem (YET, later it will be), so if you ask stuff related to getting your iPod touch connected on www.superuser.com, they can also help you. You should get ping set up anyway to figure out what's going on. Then, afterwards you might need to ask why you can't connect on superuser.com (or another, similar forum), or you can google some stuff on ad-hoc connections (might be permissions issues with Windows 7).
Good luck on this phase of getting connected.