First you need to click on the blog that you want to enable the ask on:

You then need to click on ‘settings’

Now, you need to scroll down until you find the ‘ask options’ which will look like the image below. You need to select the box I have highlighted in blue & fill in text area I’ve also highlighted in blue. You can also enable anonymous messages by selecting the box i’ve highlighted in purple. CAUTION: be aware that by enabling ‘anonymous asks’ you may become subjected to cyber ‘hate’. I would enable this if you get hurt or offended easily.

Finally, scroll all the way to bottom and click: