Mediation is voluntary so both sides will need to agree in order for mediation to proceed. If you want to try mediation, and the other side has refused, some mediators may be willing to speak to other side about mediation.
It may be a lack of understanding that has prompted the refusal. A mediator may be able to respond to concerns about the process or cost which have held back the other side.
At the end of the day, no one can be forced to mediate but sometimes a refusal to mediate is based on ignorance of the process rather than a rejection of the promise of mediation. It is worth finding out why the other side does not want to mediate to see if there is a workaround.