Go to Slimstat > Settings > Basic and set Tracking Mode to Client Side. Don't forget to invalidate/clear your plugin's cache, to let Slimstat add its tracking code to all the newly cached pages. Also, if you're using W3 Total Cache, make sure to exclude wp-slimstat.js from the minifier: our code is already minified, and it looks like W3TC breaks something when it tries to minify it again.