1. From Setup, navigate to Installed Packages.
2. Click on Manage Licences next to the Package Name.
3. Click Add Users.
Please take note on the 'Allowed and Used Licenses'.
4. Select users.
5. Click Add.
If you got an error in accessing the package, please contact the package provider, it could be due to the fact that you have used up all allotted number of licenses and may need to purchase more.