Each Mobile Install User requires a license of Mobile Install. Mobile Install licenses are different than SI licenses.
To see how many licenses of Mobile Install you have, log in to SI and click the button with your username in the upper right corner of interface:
This will open the My Account Information form. Click the Registration tab:
For every SI license that you have you automatically have a Mobile Install license.
If you need additional Mobile Install licenses you can do this from the Control Panel by opening Mobile Installers:
Click the Buy button on the ribbon of the Manage Mobile Installers form:
Mobile Install Users are set up in the Control Panel of the SI interface. Click here for details on creating Mobile Install Users.