Transfer data

Apple describes in detail the data transfer process from Windows to Macintosh using Windows Migration Assistant. This is on the support page called Move your data from a Windows PC to your Mac

If I remember correctly Migration Assistant will only transfer data from a medium which has an OS on it to another with an OS. I think the easiest way is to connect your external disk to the Mac. The Mac should be able to "see" it. Then you can easily drag the items over. If you have too many to easily do that, I suggest that you first place them all in a single folder and then copy (drag) that folder over. If for some reason your Mac is unable to connect to the external drive, you could first copy them to a Flash drive and then plug that into the Mac to complete the process.