This error occurs only during Add2Exchange Console start up because the ExchangeMapi Collaboration Data Objects is not installed or not registered correctly on the replication server. 

An error occurred while attempting to logon to Exchanger Server: EX1, Mailbox: ZADD2EXCHANGE. Please verify that the account is valid and has the appropriate permissions.

Object variable or With block variable not set Number:91, Location: 1.

Location entries are listed below.  The error location above means unable to create a CDO session.

1: - Unable to create CDO Session
2: - Cannot connect to the information store
3: - Information Store Count is 0
4: - Getting Store # of Count Failed
5: - Failure to set information store root folder
6: - Failure to get information store root folder
7: - Root folder has no folders
8: - Failure to finalize global CDO Session
9: - Failure to release CDO Session
 

The resolution is to see what version of Exchange you are connecting to.  If you have installed Add2Exchange on an Exchange 2003 box, it is possible the Cdo.dll file got unregistered in the exchsvr\bin directory.  Reregister it. Start run

Regsvr32 "Exchangeserverdrive\Exchangeserverdir\bin\cdo.dll"  Replacing the drive and directory with yours.

If this is not installed on an Exchange 2003 box and/or you are replicating to an Exchange 2007 or 2010 box, and the replication box does not have Exchange power Shell installed, you will have to install the ExchangeMapi CDO located in the \Preinstaller directory of the zipped download of Add2Exchange. 

Please see this tip:  http://support.diditbetter.com/kb/A2E_214.aspx