Starting from version 6.0.5 the aux send mechanism has been redesigned to be more flexible. Any channel (track, group, instrument) can have any number of sends to any other channel, not just auxs.
To send a track to an aux channel click on the output channel label in the track's mixer strip, right above the effects list box, then select "Add a new send" from the popup menu. A new Aux channel will be created (if none was previously present in the song) and a send to the aux channel added to the track's mixer strip.
Like with previous versions to hear the signal returning from the aux channel you'll need to increase the aux return volume from -Inf to something near 0 dB in the master channel's mixer strip.
n-Track | Create Music 