LV 6.0 uses an older set of Serial vis, different from the VISA based serial vis you found from LV7.0 and later. If you copied the vis from a later version of LV, you will not be able to find them in LV6.0.
In LV6.0 you can use the Open Serial Port vi and use the "port numner" instead of the VISA Resource Name. The older serial vis are not VISA based.