If the LCD is blank, then the cruise control master switch is off. The switch is ludicrously positioned where you can't see it from the normal driving position - if you look to the right of the steering column, you'll spot the bank of switches on the dash.
The master switch is a 3 position one. The centre position disables cruise control altogether, while up and down are the 'cruise' and 'limiter' functions (don't ask me which way is which.) With cruise enabled, a green speedo dial light will show on the dash. With the limiter enabled, the same light will be on but yellow.
With the master switch set to cruise, +/- will engage cruise control at the current speed. Subsequent presses of +/- will step the speed up and down, usually in 1mph increments, but occasionally it skips one (e.g. it goes from 29 to 31) The 'O' button disengages cruise control (as does pressing the clutch or brake) while the 'R' button resumes cruise control at the previously set speed (unlike + and - which would re-engage at your current speed.)
The limiter is similar, except that the displayed speed is the maximum set speed. +/- initially set and adjust this speed, 'O' turns the system off (no limit) and 'R' resumes the limiter at the previously set speed. Unlike cruise, the limiter doesn't disengage when you use the brake or clutch pedals. Pressing the accelerator to the floor will temporarily override the limiter - the display will flash to show that you're over the limited speed, and the limiter will automatically re-engage once you drop back down to the set speed.
The display may also flash in either cruise or limiter while driving down a steep hill - this occurs when engine braking isn't sufficient to maintain the current speed, and is a warning that you should brake or change gear to get your speed back down to your selected limit.
On a late model Laguna such as yours, you'll find that the tyre pressures are shown when a certain one of the trip computer modes is selected (use the button on the end of the wiper stalk to step through.) Earlier models only showed the pressures at start-up or if one of the tyre pressures was incorrect. If you also have the cruise/limter turned on, or there's another warning showing (e.g. low fuel or change
soon) the display will cycle between various functions. Personally, I find that feature very distracting while driving