Basically you have to participate in the new Seasons to get Dug and Charles Muntz. In some ways a Season can be considered an "event" of about 3 months. A Season will span across 3 updates to the game. During each Season you will continue to do various tasks, like you normally do, to earn happiness. Then there will be other tasks that will reset daily and will grant you a small amount of happiness for each completed task. There will be other tasks that are weekly and some that will be tied to an actual event (e.g. like the Tower Event that just started). Each of these tasks will earn you additional happiness that you can then use to redeem for rewards at various levels. There appears to be 150 levels for this Season. Each level will be available after earning "x" amount of happiness. But the happiness will start from 0 at each level. The daily tasks will grant you small amount of happiness. Weekly tasks will grant a higher amount. Then the Event tasks seem to grant even more happiness amount. That is the basics.
Now with the Season, you have the free rewards that you get by completing all of these daily, weekly and event tasks. You can scroll through the list of rewards to see what you can potentially earn. In the free level, if you can reach level 150 by the end of the 3 months, then you will win Dug. Along the way you will get gems, chests, magic, etc.
They have also introduced the Kingdom Pass, which is a paid reward system. So for the same effort, you get "bonus" rewards. For this Season, the cost is $19.99 (US). So roughly $7 per month, given the Season is roughly 3 months. With the Kingdom Pass, you can get more premium type of rewards. With this Season, that would include Paradise Falls attraction and Charles Muntz. The Charles Muntz character is available at level 75, so halfway.
It is not required to purchase the Kingdom Pass to participate in Seasons. Nor do you have to purchase it at the beginning of the Season. But if you want to get all available rewards (at both levels), then you will need to purchase it. Say you wait a month to see how you progress with the free levels. Then if you decide to purchase the Kingdom Pass later, you should get all of the rewards for the "bonus" levels that you would have if you purchased the Kingdom Pass when the Season started. So you aren't really out anything if you wait. It just may feel that you are not "spreading" the cost out across 3 months.
Hope this helps.