I've seen many posts on Peugeot chat sites without a clear solution.
The Peugeot 407 (2004 or thereabouts) has a tricky cruise control system.
In particular, if you press the plus or minus button on the stick, it just blinks at you instead of setting the speed.
Do the following.
1. Get into 5th gear (if applicable)
2. Take your foot off the clutch and or brake. Completely off.
3. Accelerate to the desired speed and hold it there.
4. Turn the cruise control dial to "on". As far as I can see, you can permanently leave it in this position.
5. Press the plus button behind the cruise control stick (on the back side).
To disengage cruise control, do any of the following:
1. Press the Pause button on the end of the cruise control stick
2. Press the brake or clutch
To speed up or down when in cruise control mode, simply use the + or - set buttons on the stick. Change speed slowly otherwise it won't work and will start blinking again. In particular, do not do this to overtake or slow down behind a truck, say. Its response time is too slow. Revert to full manual control in overtaking situations, it's much safer.