Exclamation App Problem

*Not sure if in proper section, move if need be*

Noob here so try to dumb things down for me! (:

Ok, so I followed this tut HERE.
I was able to debug it to my android for testing and it worked. So I exported it as .apk and uploaded it to 4shared.com.
Now my problem is, after downloading to my phone, I went to [Install Package] and it said "Application not installed"
What would be the problem and how would I go about fixing it?

Thanks in advance (: