You don't see your subdomains listed in the drop-down menu? I see them in mine. But it lists them like this: http://www.sub.domain.com, so it's easy to scroll past them if you have a lot of domains.
Did you just add the sub? If so, maybe it takes awhile to show up there--not sure, though.
If it's not listed, and you didn't just add it, you might want to contact support to see if there's anything wrong. If you just wanted to install it manually, you can download it at wordpress.org, where they also have step-by-step instructions.
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