You wait until the tutorial gives you a free mp pack (the adventurers) - which I recall happens at lvl 6. Or just create a mp party with your current character and gear in the mp keep.
It seems to happen a little before level 6 if you go straight through on the map. So possibly triggered by which map you clear. Right when I unlocked Diamonds of the Kobolds I was sent to MP by the tutorial (I think I had beat Shieldhaven after doing Slubgut, not sure)
You are given some a free multi starter set at around lv 5. You can also use the free pizzas given to you to purchase more sets for items/classes. Then just start playing. Even with just 2 gold chest a day, you should be amass a sufficient amount of decent items after a while. Also, unlock the 4th store asap. I think its at sp lv 12. They sell lv6 to lv16 common/uncommon items. quite a number of them are fairly competitive and/or very useful and they cost very little gold.
