There was a small problem when I tried opening the site in the Sharepoint Designer (SD). I couldn’t find the site. So after googling for a couple of minutes I discovered that SD and Forms Authentication don’t play well together. I found 2 solutions of which 1 actually did the trick for me:
The first one was when you log into sharepoint you can check the “Log me in automatically” box so the site remembers your credentials. You can than click the the File menu in IE and choose the option Open in Sharepoint Designer. This will open the current page in the designer, but not the entire site. So that’s a no go because you won’t be able to use SD to create workflows and so on, it will just function as an overgrown WYSIWYG editor for that specific page. If you don’t check the box at login and you open a certain page you will see the login page in SD which doesn’t help much…
Okay time for option 2. For this option you need some more guts and more important rights 
. You need to convert to Windows authentication. The Problem is that you don’t want to change the authentication provider every single time you want open the site in SD.So you need to extend the existing webapplication. For example On my server I have a web application running on http://myserver:8000 for the default zone. This is the one that uses forms authentication. Next I extend that same webapplication to the intranet zone and put it on http://myserver:8001. This one will get you to the same sites that exist on http://myserver:8000 but instead of using forms authentication it uses the windows authentication. You can now open the site in SD not by going to http://myserver:8000 but by going to http://myserver:8001. When doing this you will be prompted to provide your windows credentials. After that the entire site opens like it should.