I have checked the Home Page, and the Business Applications / Add applications tabs to no avail - I do not have Store Manager on my site. I am the owner of the site and so I contacted support. They gave me instructions on how to set up SM as follows:
In order to activate the store manager, please follow the steps mentioned below:
1. Go to officelive.com.
2. Sign in to your Microsoft Office Live account.
3. Click on Business Applications from the left navigation bar.
4. Click on Sell Online from the left navigation menu.
5. Click on Activate now.
If you still face any error while activating the Store Manger then I would request you to provide us with the screen shot of the error you are getting.
I did all of this and can confirm that Store Manager is nowhere to be found on my MSOL Account!