A DN is a distinguished name, common distinguished names are the bind dn (cn=root,dc=example,dc=org) and the root dn (dc=example,dc=org).
An invalid syntax means you typed one of the above wrong, or used a wrong format for your program. Note that there are NO spaces in the mentioned dn's.
Please provide a link to the program your using and your domain name, so I can give the specific settings you need. (These may vary between programs, although they shouldn't)