The reason you can't browse all of the files in your site is because the advanced file manager by default uses the folder called files in your site root. You will not be allowed to go 'above' that directory.
You can update this by making an edit to the HTMLEditor > editor > plugins > kfm > configuration.php file. In this file look for the following line of code near line 52:
$kfm_userfiles_address = '../../../../files';
If you update this path to point to the root of your site then you would be able to browse the entire site for files to link to. For example, if you have your PowerCMS loaded in a directory called powercms in your webroot you would update this line of code to be like this:
$kfm_userfiles_address = '../../../../../powercms';
Whatever folder this path points to will be the folder you start in when you click on the browse server button.