Make sure that you have a proper Wi-Fi connection established between your iPad/iPhone and the computer. Go to the Settings app, Wi-Fi, and tap the blue circle arrow next to your network (whether you're using ad-hoc or going through a router). If you see an IP assigned here, it means your iPad is connected.
When switching from a router to ad-hoc on a computer, it may sometimes it may happen that the wrong network information was exposed to the Lemur Daemon. The most failsafe solution is to hard reset both the iPhone/iPad and the computer.
First, close Lemur from multitasking by double-tapping the home button then pressing the Lemur icon until it starts to wiggle. Tap the red circle in the corner to close Lemur. Then you can power down the iPhone/iPad by holding the power button until a red slider appears, switch it to shut off the unit. Quit Lemur Daemon, Lemur Editor and restart the computer.
Restart both the iPhone/iPad and the computer, reconnect over Wi-Fi, launch the Lemur Daemon and now attempt to configure the MIDI Target in Lemur.