You shouldn't have to setup the port, ie 12345...it's only the IP address you need, of your desktop computer.
Go to System Preferences on your Mac, then Network, select AirPort, and where it says Connected look up underneath
"AirPort is connected to Snow Express and has the IP address 192.168.1.69"
Use this IP address in SKTCH Remote settings panel on your iPad.
Fire up SKTCH Desktop and you should be all good to go..
Note: your mac and iPad must be on the same WiFi network.
If you are trying it not at home or public wifi there is a big chance port 12345 will be blocked meaning you are not going to be able to use SKTCH Remote to control desktop. At home, unless you are purposely blocking ports including 12345 you will need to open that one up. Unfortunately, setups are all different depending on your router manufacturer so I won't be able to help there =[