This is build of VirtualBox app and service running on Synology NAS boxes with appropriate Intel CPU. This repo doesn't contain GUI for that head to phpvirtualbox4dsm.
Keep in mind installing this package can make your NAS unreachable since it is still in beta phase.
For building an troubleshoting check Wiki pages.
First you need to find out what arch your Synology NAS belongs to, you can consult following page What king of CPU does my NAS have. Not all arch are supported by this package.
Once you know what arch you need download appropriate package for you Synology box and install it manually in package center. During installation you will be asked few questions. If all goes well you will have VirtualBox running. For remote management access you will need PhpVirtualBox or some other application like VBoxManage for Android. Latest spk file can be found in the release page.