Whenever I'm stuck with figuring out proper LDAP usage, I revert to testing with LDAP Browser Editor Despite. Based on your code looking for sAMAccountname, plus the fact you call your variable 'ad', I'm guessing you're hitting an Active Directory server. I know that LBE will be able to hit it (it works with ours) and you can triple check your filters.
For one thing, you need parens around the filter: (dn=CN=JUSER,OU=ACCT,DC=MYCOMPANY,DC=COM). I always use dn, but distinguishedname might also be valid. Use the sAMAccountname filter to view your entry in LBE and verify that you are actually using the distinguished name of the person