I figured this out in my case. I'm running WordPress on my Windows server, so there was a permission issue when the plugin was installed. The plugin is supposed to auto-create a couple files and the cache folder but that wasn't happening. This section from the plugin FAQ did the trick for me:
Troubleshooting
If things don't work when you installed the plugin here are a few things to check:
1. Is wp-content writable by the web server?
2. Is there a wp-content/wp-cache-config.php ? If not, copy the file wp-super-cache/wp-cache-config-sample.php to wp-content/wp-cache-config.php and make sure WPCACHEHOME points at the right place. "plugins" should be "mu-plugins" if you're using WordPress MU.
3.
Is there a wp-content/advanced-cache.php ? If not, then you must symlink wp-super-cache/wp-cache-phase1.php to it with this command while in the wp-content folder. (WordPress MU users should replace "plugins" with "mu-plugins".)
ln -s plugins/wp-super-cache/wp-cache-phase1.php advanced-cache.php
If you can't do that, then copy the file. That will work too.
4. If pages are not cached at all, remove wp-content/advanced-cache.php and recreate it, following the advice above.
5.
Make sure the following line is in wp-config.php and it is ABOVE the "require_once(ABSPATH.'wp-settings.php');" line:
define( 'WP_CACHE', true );
6. Try the Settings->WP Super Cache page again and enable cache.