Assigning a license for a user can be done on the Manage Licenses page for Yoxel. Here you can select new users and remove licensed users.

Proceed to Setup > Click Installed Packages (see link on the left in the App Setup section below the Salesforce toolbar) > Click Manage Licenses underneath the Action Column > Select Add Users or Remove multiple users. 
Click Add Users to include new users to Yoxel and select who you want to give licenses. Click either Add All Users or Add button. You will only be allowed to add users up to your Allowed Licenses total. After adding a user, that user should be able to access the Yoxel tab. If a user has any issues, check their profile settings to ensure they have the correct permissions.

If you assign all available licenses, but would like to grant licenses to additional users, you can purchase more licenses by following steps in this article.

If you have installed Yoxel for Salesforce with access to admins only, users with custom profiles will see a message saying that they are "Unable to access Yoxel data" when accessing Yoxel. To ensure that these users have Yoxel tab and application access see this article.