I would like to find someone who would proof-read a Press Kit of an Mac OS X app in exchange for a license. Do you know any website / service where I could find some English native speaker, ideally technical writer, who would be interested in an offer like this?
If you offer a free license, I would ask people you know directly. Easier and more promising than online.
Sites taking licenses as payment do not exist as far as I know. But there are free services out there. Maybe you want to check them:
I haven't tested any of them. Also take a look at the Startups StackExchange site.
Places where you can find technical writers:
Society for Technical Communication - http://stc.org
Your local STC chapter.
Technical Writer Mailing List (TechWhirl) - http://techwhirl.com/
Write the Docs - http://www.writethedocs.org/
Technical Writing World: - http://technicalwritingworld.com/
Any number of groups on LinkedIn, Google+, etc.
A local college or university that has a program or even a single course offering in technical writing. Students are always looking for ways to build a portfolio of writing samples.