I have installed MS-DOS, FreeDOS and Windows 98 in Oracle VirtualBox. I want to try Commodore 64. How do I install a Commodore 64 emulator in VirtualBox?
It's not possible. VirtualBox only supports emulating IBM-compatible x86 and x86-64 systems. The Commodore 64 uses a 6510 CPU and a wildly different architecture.
In order to run Commodore 64 software, you need a dedicated C64 emulator such as VICE.
Actually, if you really wanted to you could probably install MS-DOS 6.22 and then get VICE for DOS (http://vice-emu.sourceforge.net/index.html#download) working inside the VM. I don't know how well this would actually work, but once you had your Config.sys and Autoexec.bat worked out the VM could auto-boot right into VICE. I've thought about going this route as an experiment a few times, but I've never done it because VICE works so well right on my desktop.
This page (http://www.scampers.org/steve/vmware/) has a driver pack for MS-DOS/Windows 3.x hosted in vmWare, and I am assuming that some of the driver may also apply to VirtualBox, so I pass the link along in the hopes it may be helpful.
If you go down this path, I'd love to hear how it works out.