Unfortunately the URL is a bit tricky but it's much easier in the latest version of Reports (v4.4.2). These steps should get you the URL you need in 4.4.2 (see the sample report for details):
1. Go to a post in your forum and copy the URL, it will look something like this:
http://www.dotnetnuke.com/DotNetNukeProjects/ModuleReports/Forums/tabid/971/forumid/136/threadid/66287/scope/posts/Default.aspx

2. Now, replace the numbers after "forumid" with this text "[ForumID]" (including square brackets but no quotes), and the numbers after "threadid" with "[ThreadID]" giving you a URL that looks like this:
http://www.dotnetnuke.com/DotNetNukeProjects/ModuleReports/Forums/tabid/971/forumid/[ForumID]/threadid/[ThreadID]/scope/posts/Default.aspx

If you aren't using 4.4 then its a little trickier but still doable. Here, all you have to do is get the URL for a post in the forum, just like above and then just remove everything after "forumid/" so it looks like this:
http://www.dotnetnuke.com/DotNetNukeProjects/ModuleReports/Forums/tabid/971/forumid/

So in the case of your site, you should use this URL for 4.4.2:
http://www.cube-tecforums.com/portal/Community/tabid/56/forumid/[ForumID]/threadid/[ThreadID]/scope/posts/Default.aspx
or this one for the earlier version
http://www.cube-tecforums.com/portal/Community/tabid/56/forumid/

Sorry if that's a bit confusing but hopefully that solves your problem, please let me know if you have any other questions.

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Andrew Nurse
DotNetNuke Core Team Member and Reports Module Project Lead
Microsoft Certified Professional Developer