Why cant my computer detect my second monitor?

Ok so i have been trying to us my tv as a second monitor for my computer. I watch a lot of tv episodes online from abc and fox and i want to make it to where my tv will show what my computer monitor shows. I have an HP desktop computer and a panasonic plasma tv. i have the DVI slot in back of my computer monitor that is empty and an empty HDMI slot on my tv. so i hooked the dvi end of my cable to my computer and the HDMI end to my tv. But my computer wont detect the tv i keep reading how to's online and everything says to push the FN+F5 on but that is only for mobile PC and idon'tt have a mobile PC so what do i do. I can't activate the detection mode manually because i don't have a mobile pc but even my computer help onn tells me to do this but it wont work. and yes i did chang the input on my tv so its not just set on tv instead of the dvmi setting. any help would be great thanks.