My example looks like this:
<datasources>
<xa-datasource>
<jndi-name>MSSQLXADS</jndi-name>
<!-- uncomment to enable interleaving <interleaving/> -->
<isSameRM-override-value>false</isSameRM-override-value>
<xa-datasource-class>com.microsoft.sqlserver.jdbc.SQLServerXADataSource</xa-datasource-class>
<xa-datasource-property name="ServerName">myserver</xa-datasource-property>
<xa-datasource-property name="DatabaseName">mydatabase</xa-datasource-property>
<xa-datasource-property name="SelectMethod">cursor</xa-datasource-property>
<xa-datasource-property name="User">myuser</xa-datasource-property>
<xa-datasource-property name="Password">mypassword</xa-datasource-property>
<!-- corresponding type-mapping in the standardjbosscmp-jdbc.xml (optional) -->
<metadata>
<type-mapping>MS SQLSERVER2000</type-mapping>
</metadata>
</xa-datasource>
</datasources>
Maybe you have some hidden sequences?
Or the server name could not contain \\, try / instead.
I don't know the exact syntax here :(
Maybe this wiki helps