In that case you'll need to use the command chmod or maybe chown.
chmod - sets the permissons on the file or directory
chown - sets the ownership of the file or directory
So, you can either set the permissions so that the group your user account in has read write access or change the directory that you are sharing so that it is owned by the user account you are using to logon from the Mac.
Easist way is with chown
sudo chown <account name> <path to shared directory>
by default the owner of a file in Linux has full controll of it.