While my father and I were servicing the air conditioning on my 1993 SC 400 we couldn't find the low side port. Someone on here or Club Lexus said it was 6" away from the passenger side firewall which is completely false. I looked and looked on here and CL and couldn't find any pictures or good descriptions about where it was, so I took some pictures for you guys who may be looking for it.
First off, the high side is extremely easy to see. Right when you open up the hood, it's sticking out of the plastic tray immediately to the right of the hood latch. I didn't take any pictures of that because it's staring at you when you open the hood.
The low side service port, however, is actually on the compressor itself. It is covered by a black boot. Here are two pictures that show where the port is located (we have the gauge line hooked up to it so you can see where it is):