Not sure you can. Only forward compatibility is assured.
Oic. thx Jim
it is really a bad news for me .....
On the contrary, it gives you a valid excuse to upgrade to CS5.5 on the other system.
There is no backward compatibility, only forward compatibility using conversion to the new project format, done automatically.
Basically CS4 to CS5 to CS5.5 all work.
CS5.5 to CS5 or earlier, no go.
You can export your sequence as an XML file and import it into CS5. If it is all cuts and dissolves you should be fine. Titles and effects will probably not be retained. Good luck!