.py files not showing up in Addon UI

Hello all

Blender newbie here, coming from a CAD Engineering background. Attracted by the open source nature of Blender 3d. Have installed Blender 2.54 and impressed so far. A few pains. One of them is using scripts. I have place the scripts in the appropriate folder. However, they are not showing up in the AddOn UI, so that I can enable them. These were downloaded from the net, so may not be using the current version of Python. Aware that they may not just work. Is there a naming convention or some part of the script which has to be changed?

An aside. Is there anything like Slice tool in Autocad? I have attempted to use a plane as a cut plane, Boolean difference. No success. Of course I may be way off in the mechanics of it all.

Thanks for your help