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1) If you have instaled VirtualBox skip this step. Download and install VirtualBox – Download Now (direct .dmg download link) – visit VirtualBox Downloads page

2) Launch the Terminal (located in /Applications/Utilities/)

3) Decide which versions of Internet Explorer you want to download and install. Select the text below and copy it:
  - Install ALL versions of Internet Explorer: IE7, IE 8, and IE 9
    curl -s https://raw.github.com/xdissent/ievms/master/ievms.sh | bash

  - Install Internet Explorer 7 Only
    curl -s https://raw.github.com/xdissent/ievms/master/ievms.sh | IEVMS_VERSIONS="7" bash

  - Install Internet Explorer 8 Only
    curl -s https://raw.github.com/xdissent/ievms/master/ievms.sh | IEVMS_VERSIONS="8" bash

  - Install Internet Explorer 9 Only
    curl -s https://raw.github.com/xdissent/ievms/master/ievms.sh | IEVMS_VERSIONS="9" bash
    
4) Copy and paste the selected command from above into the Terminal and hit return, this will start the download and conversion process.

5) Launch VirtualBox and boot Windows & Internet Explorer

Remember that the default Windows admin password is “Password1″, it’s also the password hint within the VM should you forget it.

resource: http://osxdaily.com/2011/09/04/internet-explorer-for-mac-ie7-ie8-ie-9-free/