easiest way I know how to do it in Nuendo 3 is using otiumFX's Compadre. it's a bit different doing this in SX2 or Nuendo 2 compared to 3, as posted above.
great a QUADRO group, and put Compadre on it.
buss your bass track to this group (i usually call it bass sidechain)
go to vst connections, group/fx tab, and on the 'bass sidechain' group, right click and add a child bus (Ls, Rs, i believe is what it adds).
go to your kick track's sends, and use a send to send it to this child bus, Ls/Rs (it'll be in the list). put the fader to 0.00 and click the little pre-fader button so it turns orange.
click the key in button the Compadre and you're off!
BUT IT'S WAY FUCKING EASIER IN PT!!!!