In Neuendo Three You have to use the temp waveform to change the temo. It allows to graphicaly edit the temp through out the song.
Select Project->Temp Track
and draw away.

Hey thanx! never noticed that oh and by the way, is there a way to make a mixdown of every bus in the mix? in Nuendo, I've only been able to mixdown one bus at a time.

File -> EXPORT -> Selected Tracks
You can use your bus as a selected track.

The manual says the following:

The channel for an audio track.
This will mix down the channel for the track, complete with insert effects, EQ, etc. This
can be useful for turning a number of events into a single file, or if you are using CPUintensive
insert effects – by exporting the track and re-importing it into the project you
can turn off the insert effect, saving processor power.