Most of the time this issue relates to an error when typing in the username for the mailbox to be migrated (or for the admin account if this option is used) so please check this first.
Does the username exist?
Check to make sure that the username exists. Usually it will be the same as the users primary email address.
To check if the account is viewable log into webmail (OWA) with a user or administration account.
Create a new email and then copy and paste the user id created within Migration Monster into the "To" field
If the address resolves to an existing mailbox in the address book then it will appear in a provided list which is the correct response.
If however it fails to appear in the address book Migration Monster will not see it.
Possible causes are that the mailbox is hidden, in which case please check this configuration within MS Exchange
Alternatively, the mailbox may have been deleted/non-existent or there may be another issue with the email system/mailbox setup.
Address book service
On versions of MS Exchange where the Address book Service exists (within services), check to make sure it is running. When stopped you will not be able to enter the address book in outlook
If the destination account is new then the language and timezone setting may not have been configured. please complete the mailbox setup
Office 365 specific
If using an admin account within office 365 please check that the account has been assigned a license. As with all office 365 changes, please allow time for the changes to propagate.