I'm trying to install an extension with Magento Connect and it says the extension is installed but I don't see anything new in my store's back-end.
This is actually not a problem with the extension, it's a problem with your Magento Connect Downloader. Most of the time this is because you or your administrator moved the Magento installation location to another directory without re-initializing the PEAR registry. This will make your Magento Connect point to another directory instead of the current Magento installation directory.
You'll need to ask your host to do this or log in via SSH and run these commands in your root Magento directory:
./pear mage-setup .
./pear install magento-core/Mage_All_Latest
Next you'll need to clean the PEAR cache files:
rm -rf downloader/pearlib/cache/* downloader/pearlib/download/*
That should be it. Try another install and confirm that your Magento Connect downloader is putting files in the right location so they show up in your install.