The output hose goes straight out the back of the pump and has a threaded fitting. The return hose goes into the side of the reservoir and has a simple hose clamp into a short pipe that's threaded into the reservoir. I don't know how you could have reversed the hoses.
To bleed the system, make sure there's fluid in the reservoir, get the front wheels off the ground, start the engine and turn the steering wheel from lock to lock a dozen times. Check fluid level and check for bubbles in the reservoir. Repeat until the fluid level doesn't change and there are no more bubbles.
A 300SDL does NOT take ATF in PS system. Use MB approved power steering fluid like Febi.