Like ISA server 2006, even though you are an administrator of the domain, you still unable to manage the ISA server or TMG. What you need to do is to ask the delegation permission to open the ISA or TMG management console
How to do it?
open TMG properties -> select Assign Roles Tab
and Then Browse the user. For example, Jane
Then choose what role you assign. For example “Forefront TMG Array Administrator”
Then click OK & Apply->Ok.
Click Save Changes.