That could be a few things.
GParted - Check that the partition is correctly made.
It can work fine, but still be incorrect (yellow triangle & !).
mke2fs /dev/sbd2 (code could be wrong, check page 1) after you partition with gparted.
Make sure your android.item.ext is correct.
Make sure all files copied over correctly.
Wait for ~30 seconds in the bootmenu so that it really finds your MicroSD card, also you must have the battery cover on and the little support closed(that thing beneath the camera cover), otherwise the card wont be "inserted" in the N900.
I have a strange problem.
After bootins have errors
not found it: ../bootmenu.d/android.ext.item: line 2
not found it: ../bootmenu.d/android.ext.item: line 3
not found it: ../bootmenu.d/android.ext.item: line 5