>Hi,
>
>Any idea of what would cause the error:
>
>

>Cannot show requested dialog.  
>Additional information: Parameter name: nColIndex. Actual value was -1. (Microsoft.SqlServer.GridControl)
>

>
>The error happens when in SQL Server Management Studio 2008 I right mouse click on Databases and then on Attach.
>
>TIA.

It is hard to reproduce on demand bug in SSMS. As a first step, try closing SSMS and re-start and re-try. If this will not fix the problem, then you can always attach DB using T-SQL.