Once your vehicle arrives the Poipet border, you need to disembark for the immigration office.
In our case, since we took the van from AIT (Pathum Thani, Bangkok) to Poipet, we had to disembark at the Poipet border approximately 4 hours after our departure from Bangkok.
I hold a Philippine passport, so I do not need any visa to visit Cambodia. My travel mate who's from Indonesia had to get her visa on arrival at the border which cost her Baht1,100.00 (Baht100.00 more expensive than the fee from the Cambodian embassy in BKK).
If you can speak a little Thai, it would be better to ask for directions to ask where the office for Visa application is located. Otherwise, you need to speak with anyone in uniform as it is more likely that they can speak a little English.
As of this writing, the Visa office is located beside the Kasikorn bank ATMs. It's quite a big and old building with sniffing dogs and guards around.
