how can i open a .ppsx slideshows in Impress?

The format is proprietary so you may have issues. If you simply want to view the file then you may be able to install the viewer in wine and open them well.
I am not sure if the ppsx is due to be included in the supported types but hopefully they (microsoft) will move towards the open formats to give more availability (doubtful)

I have OOo 3.1.1 (Ubuntu Karmic version). It recognizes .ppsx, but only to show it directly, no editing possible.
Alongside, I have installed OOo 3.2.0 (original, vanilla version). This one asks what filter to use when I try to open the .ppsx file. I select "Microsoft Powerpoint 2007 XML", and it opens as a normal, editable presentation file.
Maybe the problem is a missing filter in the Ubuntu OOo version, or the difference between 3.1 and 3.2 - I can't verify that.

Still playing around...
Another solution: In OOo 3.1.1 (Ubuntu Karmic version), I open an empty Impress presentation. I insert the .ppsx file into this presentation (Menu: Insert - File...). It opens without problem, and is editable.

You could try 3.2

sudo add-apt-repository ppa:openoffice-pkgs/ppa; sudo apt-get update; sudo apt-get upgrade

May help, remember MS Office is a close source proprietary format so the OOo guys are basically using trial and error to try and get the file type right. Microsoft have not published the file type layout so that people get tied in to their products as only MS office can truly open MS office based files and its disgusting.