In order to show accounts that are not set to expire you will need to use the below LDAP filter.

Accounts that don’t expire:

(&(objectCategory=person)(objectClass=user)(|(accountExpires=9223372036854775807)(accountExpires=0)))

 Accounts that have an expiration date:

(&(objectCategory=person)(objectClass=user)(&(!accountExpires=9223372036854775807)(!accountExpires=0)))

About the accountExpires attribute

Account-Expires Attribute

The date when the account expires. This value represents the number of 100 nanosecond intervals since January 1, 1601 (UTC). A value of 0 or 0x7FFFFFFFFFFFFFFF (9223372036854775807) indicates that the account never expires.

http://msdn2.microsoft.com/en-us/library/ms675098(VS.85).aspx