The free bus from Arinsal is the one to Arcalis, which goes through La Massana on the way, however often by the time it gets to the St Gothard bus stop it's full, so best bet if you wanted use the free bus is to take the free ski bus from outside your hotel UP to the gondola, then take the free Arcalis back DOWN the hill to La Massana.

Alternatively you can pick up the regular bus to La Massana outside your hotel - you can take either the parish bus to La Massana, or the service bus for Andorra la Vella.

The other alternative is to ski over from Arinsal to Pal, then the gondola down from Pal to La Massana.

Steve