2 possible things come to mind..
First, did you start SSMS by right clicking and saying run as Administrator.
Second are you logging into SQL as a trusted user or with a SQL login.
The reason I ask the second question is that I was unable to attach some databases when logged in as myself and I had sysadmin rights and the files were available to the SQL service user. I couldn't get it to work until I logged in as 'sa'. Thought it was weird but there it is..
CEWII