MSDN says http://msdn.microsoft.com/en-us/library/ms256086.aspx
degree[@from != \"Harvard\"] - All elements where the from attribute is not equal to \"Harvard\".
First, enclose your whole XPath with ' so that the XAML parser does not try to interpret @Type= as a syntax error. Then, use the standard XML entity " to represent a double quote:
'
@Type=
{Binding XPath='/EssenceList/Essence[@Type="Role"]'}