I found that the Balsamiq plugin (on the account tied to the email address you used to sign-up here) was not enabled. I have enabled it for you and it is all ready to go.
Just in case you want more info about the plugin:
hailstrike, I believe that you just have to access the 'Manage Add-ons' page of your OnDemand instance, select the option 'User-installed' on the drop-down menu, search for Balsamiq and Enable the plugin.
for JIRA OnDemand instances: https://YOUR-INSTANCE.atlassian.net/plugins/servlet/upm/manage/user-installed#manage
for Confluence OnDemand instances: https://YOUR-INSTANCE/wiki/plugins/servlet/upm/manage/user-installed#manage
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