That's a little Schwinn weirdness. Pretty obviously there has to be a cable stop somewhere. The Shimano was designed with the idea that there'd be either a straight run of bare cable from the DT mounted shifter under the bottom bracket (or over but that's pretty archaic) or a stop on the downtube with cable housing going to whatever other sort of shifter there was, bar-end, stem, brifter.

If you can come up with a clamp on stop or convert to DT shifters you can use one of the little plastic cable guides that shimano sells that screw on under the BB. Like this: http://aebike.com/page.cfm?pageid=30...ils&sku=FS1092