With the eDonkey and Overnet networks, files you're downloading are automatically shared (precisely, all completed chunks of the files you're downloading are shared).
The files in the \"incoming\" subdirectory are automatically shared too.
If you want to share files from other directories, you must modify the shared_directories list. Before MLdonkey 2.02-24, you must stop the core, edit downloads.ini, the restart the core. Since MLdonkey 2.02-24, you can use
share dirname and
unshare dirname to add or remove directories from that list without stopping the core. You can check what directories are currently shared with shares.
If you add or remove files from the shared directories, you can run
reshare to notify MLdonkey that some changes happened. This command is also run periodically (every hour as far as I can tell), so it's not required.
New files (or files that the core doesn't recognize) must be rehashed before they can be shared on the eDonkey and Overnet networks. Rehashing can take some time, just be patient.
Informations about the files you're sharing can be found in the generated file shared_files.ini