The easy way to do this is:
On the AD Import Entry, on the "starting from <domain>" portion, check the box for each of your top level OUs within the domain. You will have to maintain this if you often add top level domains however.
Alternatively, where it says "Import some <users/computers>" you can use an LDAP filter. An example of this might be:
(&(objectCategory=computer)(!ou=Discard))
This should import computers where the OU is not "Discard."
Hope this helps.
Joe