HI there, that's a very old version of concrete5 (from July 2009). You should consider upgrading to a newer version.

I believe the issue is that you need to be on version 5.4 or later to successfully connect to the community at this time. You wouldn't have found much value connecting a 5.3 site anyway since there isn't much available in the marketplace for a version that old.

Is this a new site you are setting up or one you've been running for a while?

Thank you Julia. My site has been running for a while. I looked at upgrading but the process looks complex and possibly won't work because it's such an old version, from what I've read on the forums. I"m not sure how I'm going to proceed ...

Thee is an add on I want to use that is compatible with my version of concrete, but I have not been able to add it because of this issue. Is there another way?

Yes, you can purchase through the marketplace, download the files, and manually install. Here's a guide for that process:
http://www.concrete5.org/documentation/how-tos/editors/download-man...

If you are interested in updating, here's a guide for older sites like yours:
http://www.concrete5.org/documentation/how-tos/developers/update-ve...

If you do want to upgrade, I suggest copying your site and testing the process on a stage before doing it on your live site.

Thank you. I was able to install using this method :)