I am still amazed with the magic of googling. Being thankful of the help I got from Google, especially this website : http://mindlinkmarketing.com/2011/03/how-to-add-your-logo-to-your-company-video-with-imovie/ here I am going to elaborate several additional steps that I did before inserting the logo. Just a token of paying forward to help other Mac users with similar questions that I had.
1. How to resize my logo to 640 x 480 without being stretch?
Screen shot (shift + command + 4) white screen (can use blank word or blank power point)
Open this screen shot in Preview. Resize this file in Preview: Tools – Adjust Size
Open logo file in preview. Copy logo and paste in the resized white screen.
Voila, you got the resized logo with 640 x 480 spec.
2. How to make logo with transparent background?
Open logo (.png file) in preview.
Choose instant Alpha for background that we would like to delete, then click delete.
As for the detail on inserting the logo inside the video using iMovie, Scott Olson has described it very clear in his post.
My background: no experience what so ever in video editing, relying on google and you tube.