The game can be played at https://dominion.games
If you have technical issues, or just want to chat about the game, please join our discord channel
Interactive support through discord is the most effective, but alternatively you can post in the support forum
or send us a message at firstname.lastname@example.org
If you previously had an account on the MF system, you're supposed to have received an email. The email contains a link you can use to transfer. Please also check your spam box. If you haven't, contact us and we'll provide you the transition link in some other way.
If you had bought expansions on the MF system, you will be granted a year of subscription for free to those sets. Those people will also get the chance to convert the remaining of this free year to the offline version once we get that up. Unfortunately this does not apply to people who bought the subscription until the end of 2016 at MF.
There isn't much official documentation, introduction or tutorial yet, but you can check out Dominion Online Unofficial FAQ