There hasn't been a version of IE for Mac since 2002-2003.
It you want access to Microsoft Internet Explorer, you need to use Bootcamp to create a bootable partition and install a version of Windows and access IE from being booted up to Windows.
I have no real clue about the exact procedures for installing Windows on your iMac other than what I posted here.
This is the only way for you to get IE.
I don't do Windows, but somebody that runs both OS X and Windows can be of more help to you.
I would suggest that rather than use Boot Camp, you use one of the virtualization tools to create a Windows virtual machine. Then you can run the VM when you need to use IE and not have to reboot.
I started off with VirtualBox, which is free and does the job.
I currently use Parallels, which integrates your Windows apps into your Mac desktop, which results in a more seamless experience, but it's not free.
A lot of people also like VMware Fusion.