Travel time from Schiphol, Amsterdam Airport is just under 3 hours, but does require 2 or 3 changes, one way ticket € 26.20 each.
Traveling from Amsterdam Central station you still have 2 or 3 changes, but the travel time will be about 15 minutes less and the cost one Euro less.
There are trains running each 30 minutes for most of the day.
If you travel in the weekend, go to the ticket window and ask for the cheapest option, as depending on when you travel there might be a weekend return or a special offer for the spring, that is a first class rover ticket for just one day for two that will cost you less than 2 one way tickets.
Find your train times on this site: http://www.ns.nl/servlet/Satellite?cid=1083234338201&pagename=www.ns.nl%2FPlanner%2Fplannerplus2stap&p=1083234338201&lang=en
For a place to stay, best contact the tourist office in Assen, there telephone number on the site is an paid number that only works within the Netherlands, so fax or e-mail and ask their 'normal' phone number if you want to talk to them, English will be no problem, I think.
Telephone number: 0900 202 23 93 *only within the Netherlands and you pay extra, but they do not tell how much.*
Faxnumber: 0592 241 852
E-mail: vvv-assen@vvvdrentheplus.nl
Their website ( http://www.vvvassen.nl/ ) is in Dutch and German, and does not allow to link to a specific page, so if you want to try to arrange your own accommodation, select Accommodaties (on the Dutch version,) you now get a menu with several options, Bed and breakfast, Bungalows en Appartementen, Campings and Hotels, Particuliere verblijven, Trekkers-, blok- en plaggenhutten are your better bets.
Campings are campsites.
Particuliere verblijven means stays with private people, I do not know what they mean by that.
Trekkers hutten, Blok hutten and Plaggen Hutten are cabins, and as only one campsite offers them, you get cabins according to the picture I saw.
The next screen asks which town, alle plaatsen is mostly just Assen or also one village near Assen, so you are safe just to select that.
Next you get a listing, select one of the list and you get details.
From there you will have to contact the people to find if they still have vacancies.
I would e-mail the tourist information and ask them to arrange something.
I have not been in Assen so I can not give you local info there.