Creating an ad-hoc Wi-Fi network if easy. Simply click on the Airport icon in the menubar and select 'Create Network'. Choose any name you like, and you can also choose to set a security password (recommended but not necessary).
If the Airport icon is not visible, go instead to System Preferences / Network / Wi-Fi, click on the Network Name drop-down menu and select 'Create Network'.
Now connect to this network with your tablet:
Note: Starting with iOS 6, it seems that the AirPort symbol no longer appears when the iPad is connected to an ad-hoc network, even if the connection is properly established. You can check if the iPad is connected:
Once you are connected, you can now send/receive OSC messages or use the Lemur Daemon to send/receive MIDI.
Using static IPs is usually not necessary. In certain cases though, IP auto-assignment fails. Sometimes you have several networks on your computer, and the netmask will be mismatched between the computer and tablet. Regardless of the reason, setting static IPs is only recommended for advanced users or for last resort troubleshooting.