https://confluence.atlassian.com/display/UPM/Installing+Add-ons
According to this link above, you can click on the Upload Add-on link at the top right side of the page of Manage Add-on. Well that "Upload Add-on" link does not exist!
You must be an administrator to do this.
Also, I suspect you are using OnDemand. That does not allow the upload of add-ons.
If you want to add plugins, you need a host who will allow it. OnDemand does not. There are a number of Atlassian Expert Partners who do hosting, and they have varying rules on plugins, costs and so-on. You can also install it yourself and run it on your own hardware, which means you've got complete control.
If you are thinking of changing who hosts your system, one small piece of advice - stick to people who explicitly host Atlassian systems, or are happy to let you have almost total control over the server. Quite a lot of providers who host services don't get involved in understanding the software, and prevent you from setting it up in a useful way.
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