I'm surprised that this wouldn't be working. I can't give a definitive answer (since I don't use Windows), but I'll do my best to go over some possibilities.
this evening I can try on Ubuntu
1. Is there a menu item in Inkscape called "Extensions>Synfig>Prepare for Export"? If there is it means that at least half of the extension loaded correctly (you can then skip ideas #2 and #3).
2. Are there other extensions in "D:\Programmi\Inkscape\share\extensions"? (If not, then I made a mistake and posted the wrong folder name)
yes, the folder is full of *.py and *.inx files
3. Does the extension work if you place it in "Documents and Settings\<Your User Name>\Application Data\Inkscape\extensions"? (you may need to create this folder)
no, the locale folder is:
C:\Documents and Settings\<user>\Dati applicazioni\Inkscape\extensions
but copying files there, does not work.
I also tried to create the folder:
C:\Documents and Settings\<user>\Application Data\Inkscape\extensions
and copy here the files, same results.
4. Is there a file called "extension-errors.log" located in either "D:\Programmi\Inkscape\share" or "Documents and Settings\<Your User Name>\Application Data\Inkscape"? If there are any errors with the extension, they would be recorded in this file and I could figure out what happened.
yes, it is in:
C:\Documents and Settings\<user>\Dati applicazioni\Inkscape
attached. It show many extention dependancy error, but seems nothing related to svg2sif
At the moment I can't come up with other possible problems. I hope this helps. If not, maybe someone at the Inkscape forums
will have better ideas.
I will also try to post the problem on Inkscape forum.