I am a new 3pi user (just get my robots with Pololu USR AVR Programmer). It seems to be 5V powered.
May I connect my PC serial port (RS232) directly to PD0 and PD1?
If I am not, what level shifter do you recommend?
Otherwise, may I use the USB AVR to communicate with my own application?
Could someone point me to usefull example links?
You definitely should not connect your computers RS-232 port to the UART on the 3pi. RS-232 voltages are typically well outside the acceptable range for a microcontroller I/O line, and such a connection would likely permanently damage the 3pi.
The USB-to-TTL-Serial adapter feature of our AVR programmer can be used to communicate between your computer and 3pi. More information on this feature is available in the Communicating via the USB-to-TTL-Serial Adapter section of the Pololu USB AVR Programmer User’s Guide: