VirtualBox is a general-purpose full virtualizer for x86 hardware, targeted at server, desktop and embedded use.
For one thing, it installs on your existing Intel or AMD-based computers, whether they are running Windows, Mac, Linux or Solaris operating systems.
Secondly, it extends the capabilities of your existing computer so that it can run multiple operating systems (inside multiple virtual machines) at the same time.
Step 1: Goto this link
Step 2: Click on Link "OS X host".
Your download will start.
Step 3: Right click on downloaded file and click Open.
Step 4: Double click on vitualbox.pkg file
Step 5: Click Continue
Step 6: Click Continue
Step 7: Click Install
Step 8: Enter System/User Password
Step 9: Open Security Preference, if you get below message. Alternative way go to System Preference -> Security & Preference.
Step 10: Click Allow
Step 11: go to Downloads and then open your file again. Repeat Step 4 to 8 if required.
Step 12: Click Close. your installation is done.