hello
I use realbasic talking to arduino in a couple of products (for example
http://www.interactiondesign-lab.com/idshop/product_rfidmonamour.html)
I should extract some code and paste it on the playground but it will take me ages... in the meantime use the example called "serial_read_basic" that you find in arduino already.
Whenever you send an H it will turn on the LED... you can use this as a strarting point.
one the realbasic side you could use code like this (this is very rough now)
given that you have an object called serial1 on your window
Serial1.SerialPort=System.SerialPort("/dev/usbserial-3B1")
If Serial1.Open then
Serial1.Write("H")
Else
MsgBox "The serial port could not be opened."
End if
this code opens the communication port and talks to arduino.
sending an H
check the help in realbasic and you'll find all the info you need
be ready to write an article for the playground about your experiments

massimo