where is the low pressure port to fill up the freon ?

It is on the intake line side of the evaporator, illustration is poor appears to be on the front side near bumper level.

A 1996 Audi A4 does not have a low pressure "port". It only has a low pressure switch that can be removed (jumper the switch connector), replaced by a 82272 Mastercool adapter that you can recharge through. I'd suggest you search a bit on the Internet, and you'll find a very useful page on how to do this.