The protocol Microsoft SQL uses is called Tabular Data Stream (TDS). Your traffic might be using a non-standard TCP port. Find the TCP stream that you think is between server and client, select a packet, right-click and select Decode As.. and choose TDS for the protocol (using the server side TCP port as the reference).
If that doesn't work it probably is because your TDS traffic is wrapped inside of SSL, for encryption security. In that case you will first need to provide to wireshark the server's private key - as per
http://wiki.wireshark.org/SSL