Attaching a database in SSMS gives error
This doesn't look like a permissions issue at all, but rather a bug in the UI.

If you are using Management Studio Express, then you should consider two options:


Ensuring you are on the most recent service pack (yes they apply to client tools as well), and perhaps the most recent cumulative update as well.
Many of the bugs in the 2008/2008 R2 version of the tools have yet to be fixed, and likely never will be. Never mind that the Express version has limited functionality. So stop using the crippled Express version from 2008 R2; the fully functional version of Management Studio is now absolutely free as of SQL Server 2012 Service Pack 1. Yes, you can manage all of your 2005, 2008, 2008 R2 and 2012 instances with the newest version of the tool.


In fact, as it turns out (thanks @Justicator), this bug has only been fixed in the 2012 version of the tools, so looks like option (2) is your best bet (or, just not using the UI for this type of task).