So in AD is the Manager drop down field populated?
The mapping AD recognizes is the full DN path of the user account so make sure you import the DN from AD into an attribute in Service Desk for each user. Once you have done this, you could create a new Data Import mapping to Service Desk and call it managers mapping. Map this to the user table and map the DN from AD to the Managers.DN attribute in Service Desk. This should give you the result you want.