The one that passes directly under Namba station, I think it's the Midosuji line. Just ride it north a few stops to Umeda, take the East exit, climb the stairs to street level, head east a block or so, and then head south into the covered shopping walk.
Osakaboi, I think I can see the Umeda shopping walk on the map.
http://www.mapfan.com/m.cgi?MAP=E135...41.50.8&ZM=11& Zoom in to the closest level to see it.
I don't see a large circular building on the Namba map. Maybe it has been torn down now? You can see it in this photo, lower right, behind the Hard Rock Cafe:
If you head straight south past the Hard Rock cafe, you can't miss it. Start circling around it to the left and you should see the shop facing northeast..