When purchasing or renewing a HoudiniESQ license you will need to install it from within Houdini. Here are the steps required.
Login to HoudiniESQ as a user with admin privileges. If you have a Solo license then you are a admin
Open the Organization manager located under the big orange arrow in the upper left hand corner of menu
Click the Licenses tab
Double-click the HoudiniESQ Seat license in the list. Verify that the product you are about to upgrade is in fact a HoudiniESQ Seat license. Here is an example Houdini ESQ 1.5 Practice - Seats 50 Expires May-06-2011
Click the Upgrade button
Click the I have a License box
Paste the license (entire Email body) into the field
Click Save
Logout then back in.
Add new Staff members if the license allows by revisiting the Organization Manager. From the Staff tab click the plus (add) icon.