As you might know GIMP is a great free open source alternative to Adobe Photoshop. Usually GIMP gives you the possibility to open and edit PSD files out of the box. But in some cases you might get the error message: "Error loading PSD file: Unsupported colour mode: CMYK". Though GIMP has some support for the CMYK work flow the opening of existing Adobe Photoshop files (.psd) that use the CMYK model unfortunately isn't possible. But there is a very easy way to solve the problem by converting the Adobe Photoshop .psd file.
The probably easiest and fasted way to to do is an online editor such as http://pixlr.com/editor/. Just go to the site, open the .psd file (File >> Open image) and save it under a different name and/or format (File >> Save).
1. Get a friend to open and save the file in an alternative format or set the colour mode to RGB
2. Use the imagemagick convert program to convert the colour space to RGB and create a PNG file. For example: http://www.imagemagick.org/script/convert.php
3. Install a trial version of Photoshop. You would have 30 days or so to register so this is quite a complex process.