If you want to go to Mersing by bus, then you can use
this bus ticket booking form. You can select your own seat and your booking is confirmed instantly.
Alternatively, you can go by
taxi. Or if you're the born to be free kind, you can
rent your own car.
Each means of transport has its own pros and cons of course. The bus is obviously more budget-oriented, whereas the taxi will offer privacy, convenience and comfort. A rented car gives you all the freedom you need to create your own Malaysia road-trip adventure.
By the way, if you're coming in from Singapore, it's good to know that one of the advantages a passenger car has over traveling by bus is that sedan passengers can remain in the vehicle during passport control, whilst crossing the border into Malaysia. This also applies when going back from Malaysia to Singapore.
This privilege is not extended to bus or shuttle passengers; instead, all bus and shuttle passengers must disembark, along with all their luggage, in order to walk to the passport control complex, follow the escalators and then walk to the passport control queuing area. This applies twice, at Singapore's passport control and at Malaysia's passport control. And again during the return trip of course.
In other words, traveling by car offers a good amount more comfort and convenience, as well as a few hours saved. Good to know.