Make sure you have the latest version (2.4) of the 727. I think that in version 2.3, the weather radar only works if you own the CS Weather Radar. In v2.4, it is included in the 727 as a free bonus.

Just Flight does not have an update to 2.4 for their boxed 727. In the Captain Sim download version, the description, in the aircraft.cfg, still says it is v2.3. The only way to tell with the download version is from the file itself (csp721_2400.exe).

What happens when you press SHIFT+7? Does the 2D weather radar show? That would be another way of finding if it is v2.4.

Mark