On a car like this, you really need the owners manual.
Call 1 800 FOR-MERCedes and they will sell you one over the phone, probably the fastest way to get it.
When attempting to lower the top, the first thing I believe happens is the windows all roll down, including those little rear ones (the little wedge shaped ones). Are the windows rolling down or not? Will the door windows roll down using the window switches?
The previous posters comments don't pertain to your SLK, the latches are all hydraulically operated on the SLK, on his cabriolet, there is a manual latch you use to unlatch the front latches.
Will the windows roll down in the express or "toll booth" mode (the one touch down feature)? If the wondows have lost synchronization, like if the battery went dead or was disconnected) this possibly may keep the top from operating.
Be very careful about this top, it is very sophisticated and if you can't figure it out, consult a repair shop, you can really mess it up if your aren't careful.