That's easy to do. On the main report, in design mode, right-click on the subreport and choose Change Subreport Links. Then in the Report Links dialog box, pick the field from each report that they will be linked on. When you are done, CR will filter out the data in the subreport so that it matches the group in the main report.

I would create a second detail section and put the subreport in the second one. That way you know that the main detail has already printed. If still nothing prints, I would check that the subreport is actually getting the correct data passed to it. Put the parameter field in the header so you can see what data is being passed. On the other hand, if the subreport isn't printing at all, then something else is going wrong. Can you double click on the subreport to open it in a new window? I had this problem once and I installed SP3 for CR and then it started working fine. So you might want to download the latest service pack if your problem is similar to what I had happen.