[Plugin: WordPress Mobile Pack] How Do I Give Permissions and Make Writable

Hello,

I am new to this blog thing. I have this plug in installed in our WordPress blog, but when I activate it I get this error:

Critical WordPress Mobile Pack Issue
Not able to cache images to /home/drblog/html/wp-content/plugins/wordpress-mobile-pack/plugins/wpmp_transcoder/c. That directory is not writable. Please ensure that the web server has write- and execute-access to it.
(Deactivate and re-activate the WordPress Mobile Pack once resolved.)

How do I give the web server write and execute access? Any help is appreciated.

Thank you!