It's not letting me put youtube videos onto my iPhone 4?

I've download YouTube videos onto my mom's computer and the type is.mp4 but it won't let me put the videos into the Movies or TV Shows folder on iTunes. I did what I was supposed to do in order to put videos onto my cell, but it's not letting me do it. I have enough space on my iPhone, too. Is there a reason why it's not letting me put videos on my cell?
DO NOT LINK ME TO ANOTHER SITE, DO NOT RESPOND IF YOU'RE GOING TO BE DISRESPECTFUL, SARCASTIC, ETC. Only respond if you know how to fix this problem.

I also have iPhone and this is the way how I transfer music/videos from YouTube to iPhone:
I use the prog from this website: http://www.dvdvideosoft.com/
(YouTube to iPhone Converter, you do not need to go there unless you like this way I'm doing for my videos)
The program downloads music from YouTube and itself transfers to iTunes.So, in fact I never need to worry why the music is not added to Itunes. The music is added as a separate playlist and then I can sync it with my iPhone. Quite easy.

I also want to download youtube video to my iPhone before, than I search on youtube and find this tutorial video about how to free download youtube video to iphone. I follow the tutorial step by step, and I can easily enjoy youtube video on my iphone now!