Follow the section Step 1: Partitioning and do exactly what it says. If you have sshd running on your N900 and are able to use a computer to copy/paste the commands, it may be easier. You need to be especially careful in the "sfdisk" command to type it exactly as it is shown there. Beware, this will erase anything that's on your SD card.
When you get to "Step 2: Installation", forget about it, and use the auto-installer instead. Now hopefully it will work for you.
Once you have NITDroid installed and working, you can then use gparted to resize the partitions (without losing any of your data), and making room for additional partitions to use for whatever you want.
In my case I used gparted inside of Easy Debian on my N900. The UI is not optimal, and it took quite a long time to move 2GB of data on the Class 2 SD card, but it worked flawlessly.
So I made sure I had everything I needed to install NitDroid first and xtarm told me I already had bzip2 and wget.
Next I tried partitioning the card exactly as described by the wiki using the recommended 14100 setting leaving me with only 1 gb of space for nitdroid.
after both events I am still getting tuck on the "Extracting NitDroid-0.0.8-Popov.tar.bz2"
Guess I'll check out the nitdroid forums for further info.
I would like to state a scenario that I am trying to create on my N900. I have an 8GB Memory Card and I would really like to have it partitioned such that one partition runs NITDroid, and one other partition will be used by Maemo to store some data. is this possible? If yes, what should I do? I am a bit hazy with the entire partitioning stuff.... well totally hazy to be precise.