1. Can't find extension.
If Magento is unable to read the module's xml file in: app/etc/modules/Extension_Developer.xml, this issue might occur.

The extension is named Extension_Developer.xml. Every one of our extensions start with "Magmodules_" and end with the extension's shortcode..

If the extension you have just installed cannot be found in the list under:
System > Configuration > Advanced > Please disable "Modules Output" and try the following:

• Check that Magento is able to read the permissions of the files

• Flush Magento’s cache and/or manually delete the contents of var/cache.
It is highly unlikely, but if Magento is unable to write var/cache, files will be cached in your system's tmp/ folder instead