I'm not familiar with Mantis directly, but judging by the error you've provided it is looking for an LDAP server.
Windows Azure Active Directory does not implement LDAP and as such cannot be a direct replacement for AD for products that uses LDAP to query the directory.
Looking at the features in the Mantis web site I can a listing of the authentication capabilies as
◾ Default Mantis Authentication (recommended)
◾ LDAP Integration
◾ HTTP Basic Authentication Support
◾ Active Directory Integration (patches available)
None of these map, by the looks of it, map directly to Windows Azure Active Directory.
Depending on your motivation and set-up you could look at deploying an Active Directory on a Windows Azure Virtual Machine.