Confirmed. I tried on my 13.04 system. Appears to be a bad zip file. Theme exports are compressed zip files with the .otz extension containing the theme folder, jpg image and xml file. A working theme export can be opened using Archive Manager to test. Perhaps try to reinstall your file compression utilities? Try from the command line (root) after updating your repositories:
<style type="text/css">PRE { font-family: "DejaVu Sans Mono",monospace; }P { margin-bottom: 0.08in; }A:link { }</style>
<font color="#ff0000"><font face="Arial, sans-serif"><font style="font-size: 11pt" size="2">apt-get install p7zip-rar p7zip-full unace unrar zip unzip sharutils rar uudeview mpack arj cabextract file-roller<br><br><font color="#000000">I'm not sure if OpenLP uses an internal file compressor or offloads it to the OS. This may or may not help but is worth a try.<br><br>Regards~<br></font></font></font></font>