You can add video from three main sources.
1) Video stored on your computer harddisk. You might have shoot it using your cell phone and then downloaded to your PC.
2) Video from youtube.com
3) Video from video.yahoo.com
Now we will discuss how to add video from this three sources
1) For adding video from your PC , please go to create new post then in the post toolbar you will find a 'Add video' button or icon , at second last position from right hand side as shown below.
2) For adding video from youtube.com , after going on this site , you have to select the video you want to add. Then on the right hand side of the screen or browser window you will find Embed and html code written there as shown below.You have to select that code and paste into your post in edit html mode.

3) For adding video from video.yahoo.com , after going to this website you have to select your video.Then below the video it is written Add to site and the html code will be there as shown below. You have to copy this code and paste it on you blog post in Edit html mode.

I hope you like this post and is useful to you.