So you configured freeradius to use ldap for authentication? correct?
That means SoftEtherVPN has no idea about ldap. SoftEther is using freeradius which in turn is using LDAP? correct?
If I understood correctly so far, then I'd try to find out which authentication method is used on Mac OS. Pap, Chap, etc.. and see if this method is supported on freeradius.
If the authentication is for example: ''chap". Then try the following on your mac after installing freeradius-tools
radtest -t chap "MY_USERNAME" MY_PASSWORD" $URL 0 "MY_RADIUS_SHAREDKEY"
Values for authentication method: pap/chap/mschap/eap-md5
Which vpnclient/vpn protocol are you using?
cheers