A number of users have reported an issue when installing Orca3D in Windows 8 with the 64-bit version of Rhino 5. During the installation, Orca3D would report that it could not find a valid Rhino 5 installation.
We have made a change to the release version of Orca3D (now Version 1.2.4) and the WIP (2012-12-18) to correct this problem. Please visit www.orca3d.com/update or www.orca3d.com/wip to download these versions.
Note that this only affects users with Windows 8 and 64-bit Rhino 5/Orca3D.