Q: Switching from PC to iMac. How to get files from ext. hard drive to iMac?

Double-click on Documentation and look for instructions, or on wd_mac_tools and see if you can use anything inside.

(132053)

Why don't you drag and drop the folders you want from the external drive to your MBPr storage?  Locate what you want and drag and drop...works very easily.

Read How to transfer data from a PC to a Mac - Apple Support

Maybe if you provide the full file name including the extension, we might be able to tell if there is a Mac application that can open that file. If there is not one then you might as well leave it on the PC.

Go back to the PC and use a different method to put the data onto the drive. If you can't do so, download the CrossOver Mac trial and then try running this tool.

(132055)

If Windows Easy Transfer is a Windows to Windows transfer program it is not going to be of much use to you with a Mac, unless you Windows installed on a BootCamp partition.  Have you tried looking at the Install log to see if that identifies what is primary on that drive?

I would recommend using Migration Assistant, all that has to happen is to have both the PC and Mac on the same network (preferably via Ethernet) and let Migration Assistant migrate the data you need. If you are not familiar with MA, please read Apple's Move your content to a new Mac - Apple Support and then download to the Windows machine Windows Migration Assistant v1.0.5.7