I will be arriving at the Huangshan North train station - how do I reach the Baiyun hotel?

Well, you may take a bus from North Railway Station to Tangkou for CNY 20. The buses are available from 9:00 to 19:00 at an hourly interval. And the trip takes around 1 hour.

Then, you can take a bus to Yungu Temple, and take Yungu Cableway up to White Goose Ridge. Walk to the lower station of West Sea Valley Cableway, and take the cable car up to Tianhai. Afterwards, walk to the Baiyun Hotel. :)

Thanks Lydia for your advice ! Are there signs in english ? Is that doable with luggage ? I will arrive on Saturday in three week, should I expect big crowds ?

Yes, there are several English signs that can help you find the right direction. You can storage the luggage at the bus transfer station. Then you can tour around the mountain area easily.
In addition, May is actually the high tourist season. You are advised to avoid going there on weekends.

So taking up luggage up to the hotel is a no-no ?

Well, it's ok for you to put the luggage at the hotel inside the mountain. However, you need to visit the scenic area with luggage. I think that it will be inconvenient.
Otherwise, you can contact the hotel staff. Maybe they can provide pick-up service and help you carry the luggage at the entrance. :)