I don't have a sample file nearby but I can give you some links.
The main configuration file is the slapd.conf. There will be a line in it as follows:
No doubt you will have changed the suffix to one that suits your network. Note that the rootdn is the manager. The rootpw (which you need to change from the default) is the manager's password.
It appears as if you are getting a invalid credential message because you are not telling LDAP that you are the manager of the directory service.
If you want to add some entries the command would be as follows:
ldapadd -f entries.ldif -x -D "cn=Manager,dc=example,dc=com" -w secret
Note that secret is the password.
If you don't want to type the password and make it visible you can use:
ldapadd -x -D "cn=Manager,dc=example,dc=com" -W -f example.ldif
It will prompt you for the password but will not show what you type.
If you are using a GUI interface there should be some place for you to enter your username and password.
Here is some documentation.