Being an admin (or root) on the local computer is irrelevant to the domain controller -- in order to reset a domain user's password, you need to authenticate to the domain controller using domain admin credentials. You can do this from OS X with the
dscl command. Here's an example of resetting the password for the user "fred" in the LDAP domain named "domaincontroller.com", by authenticating as the domain admin "diradmin":
dscl -u diradmin -p /LDAPv3/domaincontroller.com -passwd /Users/fred
-p tells it to prompt for the domain admin's password;
-P "password" would specify the admin's password on the command line, which is not recommended for security reasons. Also, if you're not sure what the appropriate nodename for the domain is (
/LDAPv3/domaincontroller.com in the above example), check the user's meta node location:
dscl /Search -read /Users/fred | grep "^AppleMetaNodeLocation"