Try following steps if they help
Go to your Central Administration website
General Application Settings > SharePoint Designer.
Select the web application that is hosting your site collection and verify that SharePoint Designer is enabled.
There can be an issue when there are web applications in farm that have more than one binding in IIS. When there is more than one, the site can't be opened. To get around this, you will have extended my web application to a new URL (host header) and used that URL to connect with SPD2010.
Also Check if Client Integration is enabled.
we launched a SMS based search engine with free of cost..
send one sms to 888-333-1864 and search for meaning,definiton,pnr status,
branch address,restaurant address,current trend info etc..
It's absolutely free!!!
Like us at www.facebook.com/orusms