how to do i transfer my pc files on an external drive to my MacBook pro?

If the files were on your PC you could have migrated them using Apple's migration manager. See this article Move your data from a Windows PC to your Mac - Apple Support

But if your external drive contains documents, music, movies etc, you can simply connect the external drive and open each folder. Drag your documents from the external drive to the Documents folder of your Mac. If you have music and/or video, it would be easier to launch iTunes and select iTunes Preferences. Select the Advanced tab and then make sure these opens are checked. Keep iTunes Media folder organized. Copy files to the iTunes Media folder when adding. Now drag your music and/or video files from the external drive onto the iTunes icon in the Dock. This will add them to your iTunes library and copy them into the proper folder.