How do I buy Mass Effect 3 DLC on PC?

In Origin, go to Mass Effect 3 and hit the 'i' icon in the lower left corner of its box. Should open a game summary page. This page will have a 'Shop for Add-ons' button in the upper right. Click it, select the DLC and hit purchase. You have to pay in points though, but can buy just enough points to cover it, so you won't be left with extra useless points.

Alternatively you can select 'Downloadable Content' from the main menu to the game to find and purchase the content. The cost is 800 bioware points, or in real world dollars, $10 USD.