ldap_modify: Insufficient access (50)

It is exactly what is says on the tin. Your DN cn=myuser,dc=mydomain,dc=com does not have enough permissions to modify the cn=config tree. And when you are trying "to specify the config database" you are using an entirely different DN, cn=myuser,cn=config, which apparently either doesn't exist or you are using a wrong password.

To do modifications like these you need to work with an account privileged enough to modify the various databases. The "admin" account, i.e. the one account that always has all privileges, is specified in the attribute olcRootDN and its password is found in olcRootPW. For the cn=config database those attributes are found in olcDatabase={0}config,cn=config and for the "regular" database, usually of type HDB, in olcDatabase={1}hdb,cn=config.

Which tutorial or documentation did you follow? It doesn't seem like you understand completely what you are doing here.

As I had a quite similar problem, I tried daff interesting solution but to no avail. Contrary to Lynn Owens I failed to add a password to the admin in olcDatabase={0}config,cn=config. I finally managed to modify my cn=config using commands like:

ldapmodify -Y EXTERNAL -H ldapi:/// -f somefile.ldif