If compilation mode in your Magento is enabled in your Magento store, you have to turn it off, otherwise your store and admin site would be inaccessible after installing the extension.

To disable Magento Compilation, go to System–>Tools–>Compilation. 

You have to turn on your configuration cache to prevent early installation, as it could be a reason of errors.

Go to System→Cache Management. 

After purchasing and download the module's zip file, unzip it into a directory of your choice.

We recommend using FileZilla to perform the installation via FTP. Enter your store's Host IP, Username and Password, then hit Quickconnect

Carefully upload the files of the module to the corresponding directory

You should refresh Magento's cache to provide Magento with the ability to register a new extension.

To prevent an “Access Denied” error, you have to re-enter to the backend. Log in with a full administrator account to get the new permissions.

Now you should check whether your website and your new module works as expected.