Ok, I ****ume you're using pak128 nightly r757 - if you have a different version of pak128, the entries may have to be different - so please tell me if that's the case.
Anyway, for r757:
below line 305 (which should read: toolbar[0][29]=simple_tool[19],,-,- )
insert the following lines:
toolbar[0][30]=simple_tool[19],,*,4
# underground slice mode modifications
toolbar[0][31]=simple_tool[15],,^U,K
toolbar[0][32]=simple_tool[15],,END,I
toolbar[0][33]=simple_tool[15],,HOME,D
With these settings, you should be able to enter the slice mode by pressing CTRL-U; and with your HOME and END keys you can change level.