If you have an external card reader that connects through USB, then transferring your videos is a simple as moving a file using the Finder.
Before you begin, ensure that the card has previously been inserted in your phone while it is powered on. This ensures that the phone's directory structure is written to the card.
Insert the memory card into your card reader and then double click on the drive to open it. Take a look at the directories and identify the folder used for videos. Usually this is called "video", "videos", "my videos", "my flix" or similar.
Drag your video file into this folder, remove the card and insert it into your phone. You should now be able to play the video. Please note that in some cases you may have to switch storage devices so that you can see the contents of the memory card.