Here's a tip for all the people who have been frustrated by this:

In order to fully share a folder (read/write), right-click on it and select Share folder. Select Share with: SMB, type in a Name: and you're (almost) done.

Here's the tricky part everybody stumbles upon: this is not enough to make your shared folder writable!

You have to right-click on the folder again and select Properties, go to the Permissions tab and check Read/Write for Group and Others.

I'm all for security on your network, but when you want to share something immediately, quick'n'dirty, without having to change your samba configuration, this is the trick

Enjoy!