How to let video play manually in powerpoint?

When you insert the video, Powerpoint should give you the option to have it automatically play or not.

(I can’t give you a more precise answer, even though I wish I could, because I’m running Powerpoint for Mac, which is very different from Powerpoint for Windows!)

IIRC, however, there should be something in the “pane” on the right-hand side of the screen that has to do with “Custom Animation” (might have to choose from a pull-down at the top). There’s a button or two on there that fiddle with the settings of the object in question. (Also, right-clicking and selecting, um, “Object Settings” (??) should give you a window that will allow you to fiddle with it.)

Wow, I’m extraordinarily unhelpful, aren’t I?

Well, it asks you if you insert it from their preset layouts. Check this out.

Edit: If you missed it out, go in custom animation. Select the video, right click and custom animation. In custom animation pane, you’ll see there Modify:Play. From there in drop-down list of Start:, select “On Click”.
The Art of PowerPoint-ing: Embedding Video
Embed YouTube Video into PowerPoint

Let me know if you have any problem.
There are basically two ways to insert a video, first as a link and second as activeX control. Link (hyperlink) will work as long as file is there on your hard drive (i.e. on the specified path). If you want to email it to somebody with video embedded in it, try activeX control.