how do I get into the deep roads in dragon age 2?

Answer 1

You can only enter the Deep Roads on specific missions. One of which is story related. The other happens near the end of Act 2, where you're asked to go retrieve two children.

Answer 2

After you do the main story missions for a time,  it'll trigger the quest.

Answer 3

you can only enter when you're doing the quest A Paragon of Her Kind