Well, I'm not sure what text editors the n900 comes with that are more user friendly (mine is still on route to me), but if you have a text editor you need to open the file it in that and use it - which I guess has complications added if the file has restricted permissions from the root user.
Try
sudo nano /etc/hildon/..(type the filename here or copy paste it)
If you know what your editor is called, try typing that instead of nano - that's just an example and I'm doing this on an N95...
Please excuse me, I don't guarantee and not even confident that this won't break the file ownership. Careful!
1. install rootsh & leafpad
2. open Xterminnal and type "sudo gainroot"
3. Then type "leafpad /etc/event.d/hildon-control-panel-personalisation"
4. Then make change "start on started xomap" to "start on MOUNTS_OK"
5. Click the top Menu and select "Save"