Here is my server.xml entry (with substitutions):

<Resource name="jdbc/sybase" auth="Container" type="javax.sql.DataSource" />
<ResourceParams name="jdbc/sybase">

Here is the JSP scriplet I am using to test it:

try {
Context ctx = new InitialContext();
Context envCtx = (Context) ctx.lookup("java:comp/env");
DataSource ds = (DataSource)envCtx.lookup("jdbc/sybase");
Connection con = ds.getConnection();
} catch(Exception ex) {
    out.println(ex); // print exception

When I execute this page, I get this:
javax.naming.NamingException: Exception creating DataSource: tyrex/jdbc/xa/EnabledDataSource

In my Tomcat console I get a stack trace that starts with:

TyrexDataSourceFactory: Cannot create DataSource, Exception
java.lang.NoClassDefFoundError: tyrex/jdbc/xa/EnabledDataSource