if the file you zipped (with winrar) from scratch doesn't work when renamed as .pk3 I would verify the following;
1) The file you created was indeed a zip file and not a rar file ( or a rar file renamed)
2) The directories stored in the zip file are relative path i.e. \bots
3) The bot files are correctly named (not 8.3 limited) no password, etc.(check your winrar default compression profile if it exists).
4) And if all else fails try again with winzip.
you can also use the empty zip file
fritzXX.pk3.zip to make your pk3 files