Can you please check in the My Connection dialog, under Role column, what role does it shows?
If it shows as Administrator, then selection of that site, Administrator button should be Enabled. Or you can re-edit your connection to to join as Administrator to enabled HMTL Snippet option.
Hope this helps.
1) Select the site you want to Administer and click on Edit button in the My connection dialog.
2) Click Next button till you reach the Role Information dialog.
3) In the Role Information dialog, select the Administrator role (If you have permission).
4) It will prompt for the Administrator password (Please enter the same and proceed).
If you don't have the Administrator premission, in the Role information dialog, it shows the Email address of the Site Administrator, you can contact him to set the permission for you.
Hope this solves your problem.