The 91 318ic's do not have a power top, or any "buttons" or "switches" to put the top down or for the rear top cover to open - it's all a manual effort.
If you are having problems w/ the convertible top cover rear release handle on the driver's side to pop open the convertible top cover, you have to replace that handle in order to get those cables to work properly. One thing you can try is, remove the handle (very easy to do) and remove the cables from the back of it. Once you have the cables removed from the handle assembly, get a pair of pliers and pull on the cables enough so they may release the latching mechanism for the top cover. Once they release the latch assembly, you should hear a distince "click" and the top cover will pop up about 2". When the top cover pops, there is a small safety lever under the cover on the driver side, you have to depress this to totally release the top cover, then you can flip it all the way open.
You can buy a new rear convertible top release lever from BMW. Also,
http://www.r3vlimited.com, if you go into the tech forums and search "convertible", there are many threads on how to replace that release lever. It's also well known that the release lever is nothing more than a passenger side front door release handle that is inverted (upside down)... So, if you have a local junkyard w/ E30's, you could also grab the passenger side front door handle and use that as your replacement (or buy new, or search the forums for someone parting a vehicle and grab a used unit).
The steps for putting down the top should have been included in your driver's side sun visor - there should be a black instruction sheet stuffed into the top of the visor (flip it down). Put down all the windows first, then at the top of the windshield in the interior, flip down the L & R handles, once they are flipped down, turn them inward to release the top. Once the top pops up, fold the top portion rearward, then flip up and fold the rear window portion towards the front, release your top cover lid, fold the top into the top well, make sure all of the top is in neatly, then carefully close the top lid over the top and press down until it clicks (this secures it).
Hope that helped!