27 pin Arduino Microcontroller
On the pcb I have, i see only 27 pins for the microcontroller

This is very hard to believe, unless some idiot designed the board.  Even if only 27 pins are actually used in a circuit, any competent board designer will put down the pads for all 28 pins.  Pads are not only for making electrical connections, but also to mechanically anchor the part.  Granted, holding something like a 28 pin SOIC package by 27 pins should be good enough mechanically, but there is no downside to placing all 28 pads.

If the board really only has 27 pads for your 28 pin chip, then bend up the one pin that doesn't have a pad and solder down the rest normally.

Added

Wow, that really is a 27 pin footprint for a 28 pin part!  And, it's thru hole, and gold-plated.  Find out who designed this and stay away from anything else he's done.  The term gold plated turd seems particularly relevant here.

In the mean time, just bend up pin 28 and solder the package normally using the other 27 pins.  The reason to bend up the pin is to make sure it doesn't contact anything on the board.  There seem to be vias there without soldermask, so the pin could short against them.  Even if the area below the pin was covered with soldermask, I wouldn't want the pin touching it and possibly eventually cutting thru the insulation.  Bend up the pin, or even break it off altogether.