Using the Apps application it just returns a Failed status. From the command line using /usr/bin/tce-load -wi virtualbox-ose-additions.tcz the following is returned:
Downloading: virtualbox-ose-additions-modules-3.0.21-tinycore.tcz
wget: server returned error: HTTP/1.1 404 Not Found
Connecting to distro.ibiblio.org (152.19.134.43:80)
wget: server returned error: HTTP/1.1 404 Not Found
md5sum: virtualbox-ose-additions-modules-3.0.21-tinycore.tcz.md5.txt No such file or directory
Error on virtualbox-ose-additions-modules-3.0.21-tinycore.tcz
I'm a linux newb and even newbier with TinyCoreLinux, but it looks to me like it's trying to pull the wrong/older files from the repo. Additionally, and this may be unrelated, but the file virtualbox-ose-additions.tcz.dep (I'm guessing it lists the dependencies?) references virtualbox-ose-additions-modules-KERNEL.tcz, which doesn't appear to be in this TCE repo (4.x/x86/tcz).