Usually it's pretty simple and works. However if the mechanism gets stuck or the toilet can't be opened from the inside of the vehicle check if's correctly fittet. The cassette has a plastic cover that can be removed and if it's installed the wrong way the mechanism inside will not open and close it. 

Our vehicles are equipped with a Thetford Cassette. Please read the manual for further instruction (attached)

Please try to install it correctly to make it work. 

If none of the above works here is a trick: Use the toilets at the swimming pools, gast stations or Restaurants. Sometimes it will cost a coffee or a dime but it's a solution. The other solution would be to check the local stores (BYKO, HUSASMIDJAN, BAUHAUS, KAUPFELAGID) or Gas Stations if they sell portable toilets.