The version you are trying to install was apparently written for Mac OS 9 or earlier, hence the reference to the Classic environment. The Classic environment is an emulation of OS 9 under OS X. However, Apple did not start using Intel processors until several years after the introduction of OS X, so they (Apple) did not develop an OS 9 emulator which will run on Intel based Macs.
You cannot run LV 6 on an Intel based Mac. This is an Apple issue, not a National Instruments issue. LV 6 runs fine under OS X on a PowerPC Mac, as do several other versions.
You will either need to get an older Mac or a newer version of LV. I am not sure which is the earliest version which will run on an Intel Mac.
Lynn
Using LV on the Mac since version 1.2