When coming from overseas the closest airport is Geneva. From Geneva airport the drive takes approximately 1.5 hours. When driving from Geneva it is best to come through Switzerland on the motorway, rather than through France through the mountains. Even though it is shorter in terms on km, the route through France takes at least 2.5 hours. If coming from the Fribourg direction you can either follow the motorway A12 down to Lac Leman, then take the A9 direction Aigle and exit at Aigle, or you can drive through the mountains past Gruyères and Chateau d'Oex, or a slightly longer route through Gstaad. The mountain routes take 10-20 minutes longer, but are much more scenic.
If approaching on the motorway, exit at Aigle and follow signs to Diablerets from there. Once you have left the motorway it takes about 25 minutes to get up to Diablerets.