How do I enable Tumblr's Ask and Submit features?

Ask and Submit functions are both enabled in Tumblr's general settings menu. After logging in, click on the gear icon at the top of the dashboard. Select the blog you'd like to add the features to, and simply check (or uncheck) the corresponding boxes.

How come I'm unable to ask other Tumblr users questions?

there's a common Tumblr-side glitch which can prevent you from asking other people questions. Contact Tumblr's support team directly if, despite your best efforts, you're getting ask-blocked.