With the release of Haiku R1Alpha1, VirtualBox is a great way to play around with the alpha without committing a real partition to it.
But VirtualBox tends to act like an island, entire unto itself. Your OS is sitting on a .VDI format virtual disk that no other OS or program has ever heard of. So you’ve downloaded some new apps from Haikuware while you were in your Host OS. How are you supposed to get them into Haiku? What is needed is some format that Haiku, VirtualBox and the Host OS all know and understand.
It exists. It is called .ISO (or .CDR on Mac OSX systems). If you are using Mac OSX as your Host OS, the following Terminal command will take a folder (/home/username/Binaries Storage/HAIKU – Storage in this case) and create an .ISO image that VB will accept and that should be readable by Haiku:

hdiutil makehybrid -o ~/Library/VirtualBox/HardDisks/Storage.iso ~/Binaries\ Storage/HAIKU\ -\ Storage/ -ov

Your pathnames will be different, of course. You can put that command in a Mac OSX shell script. You can even make a clickable app from that script using Platypus. Now inform VirtualBox that this is the image you want to use as your CD/DVD. The next time you boot into Haiku, there will be a fake CD for you to mount containing all the data that was in that folder.
Mac OSX will also mount the .ISO file. In fact, it will mount it read/write. But do not have it mounted in both Host and Guest OS simultaneously! First unmount the image in Haiku, then unmount it in VirtualBox (Devices | Unmount CD/DVD-ROM) – you do not have to exit the Haiku VM to do this. Now you can safely mount the image in the Host OS, or recreate it using the command above, then reverse the process to get it mounted in the Haiku VM again.