All the products sold on CodeCanyon are allowed to be used on a single website only.
Therefore, ConvertPlug can be used on only one WordPress installation. You can’t use the same license for another WordPress installation .
If your subdomain has different WordPress installations than your main domain, you need to purchase another license for that site .
You can purchase another license of the plugin from here – http://codecanyon.net/item/convertplug-wordpress-popups-plugin/14058953
In case you wish to use a lead generation plugin on multiple websites, you could consider Convert Pro, an advanced version of Convert Plus that comes with a drag and drop editor and lots of advanced features.