There should be a bolt in the disc with an allen end. not sur eon the exact size.
Tighten it and it will tighten the rotor down.
At least thats how it is on my benz.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.