Deleting the .ds_store file will do no harm. This is only used by Finder and Spotlight to store meta data. If it does not exist in a folder then it will turn up sooner than later.

When you plug the USB cable in to the Mac from the N900, the automount daemon will normally create a mount point(s) for it in /Volumes and icons should appear on the desktop. You should also see the device in disk utility. If the MAC does not do this then my bet is that your N900 file system is corrupt (probably because you yanked the USB cable without ejecting first).