In the file /etc/shorewall/rules, you should see 4 rules, that start like this:
ACCEPT masq fw tcp
ACCEPT masq fw udp
ACCEPT fw masq tcp
ACCEPT fw masq udp
Add ports 137-139 in the list of allowed ports, mine looks something like this now:
ACCEPT masq fw tcp domain,bootps,http,https,631,imap,pop3,smtp,nntp,ntp,137,138,139,ssh -
ACCEPT masq fw udp domain,bootps,http,https,631,imap,pop3,smtp,nntp,ntp,137,138,139,ssh -
ACCEPT fw masq tcp 631,137,138,139 -
ACCEPT fw masq udp 631,137,138,139 -