GIMP Plugin Registry

Hello!

I recently started to write some plug-ins/scripts for gimp. First I would like to convert some old edit and file load plugins which I wrote for some graphic program ~ten years ago.
After looking into some other plugin sourcecodes I was able to get a running version for some of the edit modules, but i have one problem:
How can I define in the most easies way the position in the picture where the the effect is working onto (central point of a circle, for example). Sure, if I call the procedure from the gimp shell, i can just give the coordinates as parameters, but how can i do that in interactive mode?
Second problem:
For the importers I would like to have a good example where i can start with. A well documentation example code would be quiet helpful.

Tschau,

Bjoern