I can't seem to find the right spot in the desert for the transporter to work. Need a litlle help

It can be anywhere as long as you aren't in an interior. For example, you can't do it in the Lucky 38, or the Strip, or in Freeside, but you can do it at the Gun Runners.

You can use the portalponder if you meet all of these conditions:

1: You are not in an interior
2: You are in the Mojave Wasteland worldspace (Not Freeside, the Strip, Boulder City, those are their own worldspaces.) OR you are in the Big MT worldspace.
3: You are not in combat
4: You do not have any followers accompanying you. (You must fire them, setting them to wait will not work)