The crank pulley is the big pulley at the bottom of the chain that the belt runs round. Looking through the right hand (with reference to the drive sat in the car) front wheel arch with the plastic cover removed this should be clearly seen. To the right of this pulley is the tensioner.
I doubt the tensioner is faulty as if if had failed then I suspect the belt would come off. Your belt sounds like its badly worn so the first job is to replace this belt. Take the road wheel off to get better access. Put an axle stand under the subframe to hold the car. To release the old belt use a suitably sized socket on the tensoner and turn anti clockwise to pull the tensioner off the belt. Hold it there and slip the belt off the crank pulley. You can release the tensioner now. Carefully remove the old belt taking care to plot the path it takes so you can fit the replacement correctly. Fit the replacement belt, starting from the top of the engine and working down. Finally pull the tensioner back using the socket again and slip the new belt over the crank pulley and under the tensioner. Check the belt is centered on all pullies as you let the tensioner take up the slack. Might need a helper to check all is well while you do this.
For your information.
The aux drive belt drives the alternator for electrical power generation, the power steering pump, which also includes the coolant pump and the air conditioning compressor. While the operation of these items is essentual for the car it doesn't matter if any slip occurs between the crank pulley and any of the other pullies. It is this slippage you are hearing.
The timing belt(s) for there are two on the L series. The first one is within the plastic cover beyond the aux belt. It links the crank shaft to the cam shaft and uses a special toothed belt. The second one is at the top of the engine but to the left hand side (again with reference to the drivers position). It links the cam shaft to the fuel pump and again uses a special toothed belt. These two timing belts must under no circumstances be allowed to slip. At the least a slippage will stop the engine from running or make it run rough. More likely a slip will allow the valves to collide with the pistons, which normally destroys the engine.
Under no circumstances venture into changing the timing belts unless you absolutely know what you are doing. Entrust this to an expert.
I hope this helps.
Dave