Once you are in Manhattan you could actually take a bus to the stadium.
Or you can take a Northeast Corridor train from Penn Station Manhattan (34th Street) to Secaucus Junction and then change to a direct link train straight to the Meadowlands. My daughter did this last fall when the Bruce shows were at the old Giants and she was able to meet up with us with no problem.

Here is what I posted for someone else for the last batch of shows:

You can take a Northeast Corridor train from Penn Station Manhattan (34th Street) to Secaucus Junction and then change to a direct link train straight to the Meadowlands. This link just opened last fall and it's great. Here is the announcement and specifics:

http://www.njtransit.com/sa/sa_servl...&NoticeId=2168


There are also buses that leave from the Port Authority Bus Terminal (41st Street & 8th Ave) and will go there also.

http://www.njtransit.com/sf/sf_servl...edulesP2PFrom#