LDAP Connection Problems

apache and php logs location depends on your OS and if linux on distribution.
On debian like it should be on /var/logs/apache2 if my remenbering is correct.

1 / How I can setup the LDAP to check only the user in this Noeud : OU=Users,OU=TLS,OU=FRA,DC=blemea,DC=bausch,DC=com

This should work.

You have to check that ldap user (defined in global parameters) correctly connects to this base DN. (check PHP ldap_bind documentation for help)
Other hint : uncomment the debugLog calls in User->authenticate(). This can give hint to unsuccessfull binding.

Maybe you can try to play with user filter criteria check PHP ldap_search documentation for help).
Concider that this search is done like in any other tool. For instance, you can try this filter in LdapBroser to see if it returns what you expect.

2 / How I change a User created manually to become a LDAP user ?

You must update directly in database : set isLadap to 1.