Unity Free license doesn't support Native Code Plugins right now.

This limitation is also preventing publishers in the Asset Store to sell their plugins to the whole Unity Community, meaning that Unity is losing 30% of those potential sales too.
I think this limitation has no sense, because a Unity Free user probably doesn't have resources to pay $1500 for a Unity Pro license, but will more often pay ~$100 for a specific plugin he needs for his projects.

Please, Unity Team, think about it, it would make Asset Store Publishers happy and Unity Free Users happier, besides you will earn more money