Genymotion, "Unable to load VirtualBox engine." on Mavericks. VBox is setup correctly

I had the same problem and solved it by running the following command:

sudo /Library/StartupItems/VirtualBox/VirtualBox restart

In later versions, the command is

sudo /Library/Application\ Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh restart

Make sure you've unblocked VirtualBox's kernel extensions in System Preferences->Security and Privacy->General (You'll get a popup when you install VirtualBox).

Eventually, you might not have anything in your /Library/StartupItems.

Using the following command helps :

sudo /Library/Application\ Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh restart

It worked for me on two different Mavericks installs.

Update: Genymotion's 2.5.1 release (https://www.genymotion.com/#!/release-notes/251#251) seems to have fixed this issue. (thanks for the heads up @Roger!)

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For those that may be stumbling upon this a bit later, I resolved this by installing VirtualBox 4.3.28 (https://www.virtualbox.org/wiki/Download_Old_Builds_4_3). The new 5.0.0 and 4.3.30 versions didn't work for me with Genymotion 2.5. None of the above solutions worked :(

It's also worth noting that at the time of writing, Genymotion's FAQ states the following:

However, for performance reasons, we recommend using version 4.3.12