There are three ways to find you Support Entitlement Number (SEN):
Tip: If you are using GreenHopper, the GreenHopper license appears above the JIRA one. The JIRA 'license details' link is towards the bottom of the list on the left hand side.
Access the JIRA license page, which indicates your Support Entitlement Number (SEN). See Updating your JIRA License Details for details on how to access this page.
Your Support Entitlement Number is available from the licenses page after logging in to http://my.atlassian.com:
Your Support Entitlement Number (SEN) also appears on the third page of your Atlassian Invoice.
See Finding Your Support Entitlement Number in the support space for more general information about how Atlassian Support uses this number.