If you haven't updated you BIOS to the hacked version do it first
http://homeservershow.com/hp-proliant-n40l-microserver-build-and-bios-modification-revisited.html (same BIOS for all 3 versons ), this will allow the SATA to run at full speed (once you enable it in BIOS)
You will also need a 5.25 inch to 3.5 inch bay converter to mount the hard drive it the top optical bay.
Also needed are a SATA cable, I used a 60cm one but I think a 45cm one would be ok and a lot of people recommend you use one with a 90 deg connector.
Lastly you need a MOLEX to SATA power connector as hardly any harddrives use MOLEX power connectors these days but that's whats in the microserver.
On the left hand side front of the motherboard you will see a SATA connector (close to the fan connector), just plug the cable into there and I routed the cable the same path as the fan cable to the back of the optical bay.
Apart from changing the new setting in the hacked BIOS to 3mbs I didn't do anything special and the server booted from the drive without any problem even though like you id moved it from one of the front bays.