Transmission cooling lines will not go into replacement radiator?

Okay, the OE lines on this are the push in quick connects, correct? I assume they will not push in and clip into place with the retainers blocking them. On most of the aftermarket units you can access the clip using a thin screwdriver on the outside of the fitting to remove the clip. Will the clip out check the inner bore of the fitting for any damage or burrs. Now apply a bit of transmission fluid to the o-ring and line it up square and push in. Once you have it started in the bore you can usually seat it fully. Then install the clip you removed to retain it.
The clips normally look similar to the images attached.

The lines should just push in, it's possible they put the wrong fittings on the radiator, do you still have the old one? Maybe measure both to see if they are wrong or if you can swap them over to make it work. There are a lot of different sizes of tubing but the most common thing would be if they installed an SAE sized connector when the Benz uses metric.

Look the old and new units over. Some can be swapped while others are assembled so that the fitting holds the cooler in the end tank, on those you cannot swap the fittings. It also depends on who made the replacement. There are a lot of companies that make the fittings, and some are not interchangeable. Can take a close picture of both of the fittings and the end of the line, then post them on here. I can likely tell you if they can be swapped.