Can I add videos which were made by somebody else!For example I downloaded video from Youtube, will it be legal to add it here?

well its okay because fanpop doesnt actually HOST the videos they are just linked. you dont download it to your computer and upload it here (which would be sketchy if you didnt make it).
you post them as a link and any video can be traced to the original hosting site (youtube for example) by clicking the little blue arrow next to the title (see image).
thats why- if i really like something- i always click on that link and comment and rate the original as well as the fanpop content but since not every one does that i think its good to credit the video maker.
p.s. if the creator doesnt want their videos to be posted elsewhere they will disable embedding so you can only post it as a link and not a video thus bringing the viewer to the original content.

yes you can, but you have to mention who made the video, or else everybody thinks you made it ;)