These water softeners are really quite simple but they rely on "O" rings for seals to the system. These seals are also working on plastic components which do have a tendancy to wear.
There is an electronic control box which is basically a timer, activated by a count-down system signalled by the flow meter built into the water piping.
Water leaks are inevitably from the failure of one of the "O" ring seale, and the most likely one is on the cross bleed manual control just behind the solenoid valve. This valve may be used to adjust the water hardness, but it can be turned off completely which will probably stop the leak. All "O" rings are likely to leak, especially from the mail control valve which is operated by an electric motor through a reduction gear. The leaks here are caused by the poor design in the manner in which the "O" rings are employed. The only real solution is for all of these "O" rings to be replaced and although this can be done by dexterous application and intelligent ability it is best left to competence . A complete set of "O" rings is available from Waterside but a bit expensive. These are standard size seals and buying them directly from an "O" ring supplier is much cheaper, butyou will need to identify the sizes and material first.
The simplest solution is to call in the approved service emgineer from Waterside.
In the meantime turn off the supply and return valves to the unit, and open up the bypass valve to allow water through without going through the softener. (In this condition the water softener can be removed for maintenance.)
MC 250 are good usually reliable units which will automatically work for many years unattended except for the addition of the salt pellets from time to time.