In your mambo config file you can change the default /tmp directory to something else. Just follow the mambo instructions, it is all there.
I have installed mambo, there was no trick to it, but just a hint if the ownership of your files gets screwed up by editing and ftp'ing then just go into the server-manager and reset the permission to write=group,read=group (group being the people you want to have write access to the website) and then it will be fixed.