You can connect your iPad with your laptop over Wi-Fi either by having both devices connected to the same router, or by creating an ad-hoc Wi-Fi network.
Connecting through a router is often the easiest solution, as most people already have a router configured for internet access in their home. Simply make sure that both devices are connected and it will probably work. Latency will be a bit higher than an ad-hoc connection (sometimes up to 200ms or more), but depending on your usage this might be acceptable. Connecting through a router can be problematic if you have a firewall active on your router or laptop. In that case you should use an ad-hoc connection.
Creating an ad-hoc Wi-Fi network if easy. Simply click on the Airport symbol in the menubar and select 'Create Network'. Choose any time, and set a password if you want. Now connect to this network with your iPad by going to the Settings app / Wi-Fi tab. Starting with iOS 6, it seems that the AirPort symbol no longer appears when the iPad is connected to an ad-hoc Wi-Fi network, even if the connection is properly established. You can check if the iPad is connected by tapping on the blue arrrow for the Wi-Fi network you're connected to. If you see an IP, this means your iPad is connected. You can now send/receive OSC messages with Lemur, or use the Lemur Daemon to send/receive MIDI.
Depending on your version of Windows, the steps to create an ad-hoc Wi-Fi network may differ.
Windows 7: http://windows.microsoft.com/en-US/windows7/Set-up-a-computer-to-computer-ad-hoc-network
Many users opt to use a program such as Connectify Hotspot to manage network sharing on Windows.