This is just like ofArduino except that instead of using connect(portname, baudrate) you should use connect_tcpip(string ipaddress, int port)
Under the hood, it uses ofxTCPClient (you'll need to include ofNetwork) to handle sending and receiving bytes.
I use this to communicate with Arduino+Firmata devices connected to a WiFi network (in my case, using Roving Networks RN-XV devices available from Sparkfun.