Bad news i am afraid, the NowTV App is no longer available in the PlayStation App Store for the PS3
So if you haven't got the NowTV App already installed or you have recently deleted the App on your PS3 you won't be able to get it.
I am also disappointed with this news, has i also own a PS3
Here's a list of supported nowtv devices.
Took the information from this help article (see screen shot below) since my post somebody on the forum has found a workaround by going into your Apps history on the PS3 to retrieve it or something along those lines (there is a thread somewhere on the forum that talks about it in more detail).