Steinberg has announced that its installers are not currently compatible with Mac OS X Yosemite, but that they have a workaround in place.
Here are the details:
If you are planning to update to Yosemite this fall or to take part in the Yosemite beta-testing program, please be aware that our software products as are cannot be installed on this operating system. Current Steinberg software installed before updating to OS X Yosemite is not affected.
The solution we offer is a small software tool currently available as downloadable pre-release version from our website which will enable a seamless installation of our existing software range on the OS X Yosemite operating system.
Check the Steinberg site for their latest information on compatibility.