You can edit /etc/samba/smb.conf or use SWAT (Services->Network->smb->Configure->Advanced; you might want to install the samba-doc package first, so you will have SWAT online help).
force create mode (S)
This parameter specifies a set of UNIX mode bit permissions that will always be set on
a file created by Samba. This is done by bitwise ´OR´ing these bits onto the mode bits
of a file that is being created. The default for this parameter is (in octal) 000. The
modes in this parameter are bitwise ´OR´ed onto the file mode after the mask set in
the create mask parameter is applied.
The example below would force all newly created files to have read and execute
permissions set for ´group´ and ´other´ as well as the read/write/execute bits set for
the ´user´.
Default: force create mode = 000
Example: force create mode = 0755
force directory mode (S)
This parameter specifies a set of UNIX mode bit permissions that will always be set on
a directory created by Samba. This is done by bitwise ´OR´ing these bits onto the mode
bits of a directory that is being created. The default for this parameter is (in
octal) 0000 which will not add any extra permission bits to a created directory. This
operation is done after the mode mask in the parameter directory mask is applied.
The example below would force all created directories to have read and execute
permissions set for ´group´ and ´other´ as well as the read/write/execute bits set for
the ´user´.
Default: force directory mode = 000
Example: force directory mode = 0755