I presume your blog is now loading blank. First, delete the powerpress folder in your plugins folder so your blog will load again. Make sure you can sign into your WordPress blog afterward before continuing with the instructions below.
Try re-uploading a fresh downloaded copy of powerpress, and upload to a new powerpress folder in your plugins folder. Double check that the file permissions are correct.
Now try activating the newly uploaded version of PowerPress. If you get a blank page again, then something may be conflicting. Please email me at cio [at] rawvoice.com the list of other plugis you have installed, it may be that a plugin is conflicting with PowerPress.