ok,
In this case, 1st add your subreport in main report,
when you will put subreport in mainreport, you will see name of the subreport on mainreport,
now right click on sub report & there is option called create link,
and then one form will display , there you can set the link of sub report with main report,

Ok tell me your sub report content company name or other data,
if your subreport content company name then you don't have to set link between then,

if your subreport content data, which is related to mainreport, then you have to create link between them,

& make sure that both reports showing data properly before merging it,

and
Tell me it's showing any error or not showing data ?