Download the source tarball and extract it to a directory accessible by your webserver and edit config.php to match the server and channel you are using in ii.
You have to make sure to set the filesystem rights of ii's files and directories correctly so that your webserver user is able to access them. The in-file has to be writable and the out-file has to readable by the webserver user. Note that this is usually not the user that is running ii. Please read the included README for detailed instructions.