Yes, there's a separate KL locator but klm.com will also accept the Delta locator. Works either way.

Sorry I didn't come back. The next day, got an e-mail from KLM:

"Seats cannot be preassigned on inter European flights.Sorry for the inconvenience.Thank you for using the KLM E-Services"

MZtery: 1) my Delta reservation number worked on KLM for USA-Europe flight seats assignment 2) I don't think a phone call would help if the e-mail said NO.

Thank you, everybody, who helped or tried to help. What would I do without this website :)