Right.. To use as a removable drive you can do one of two things:
1: plug in your ipod nano 3G into your computer (which I assume has the latest version of itunes)
Open itunes
Wait for itunes to recognise your device
On the left hand side select your ipod (its under a header for ipods)
Now you have a screen where you can see your ipod (or a picture representation) and the status of your ipod (memory, version, serial number, etc)
At the bottom, above the memory bar are 4 check boxes
Select the one which says use as a removable storage device (or words to that effect)
Then in the bottom right hand corner click apply
Your ipod should synchronise and you can view it as a removable mass storage.
2: be warned, this is all done from memory so I cannot be sure what this does, it will either put you into disk mode or boot utilities setting
2A: press and hold the MENU and CENTRE buttons. This will reset your ipod.
Then press and hold MENU and PAUSE/PLAY buttons
2B: press and hold the MENU and CENTRE buttons. This resets your ipod. As soon as it turns off press and hold the PLAY/PAUSE and the CENTRE button.
Result:if you have a screen mentioning disk mode, plug it in and it will appear as a disk drive
If you have gone onto a screen with a lot of testing options press and hold MENU and CENTRE and it'll reset
post your results as this is totally dependant on my memory xD