Now I may be missing something obvious, but there is Day 1 DLC for Mass Effect 3 on PC, and I can't see any way of easily buying it. Despite the latest Origin update which claims to allow the purchase of DLC from within Origin, it doesn't seem to be listed anywhere in there. Ideally I'd like to pay in real money rather than by buying points in irregular quantities.
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.
Yeah i was wondering that too... but the easyest way I found how to do it was ingame, then you click on downloadable content. And it should say buy DLC "from ashes" . I payed, closed ME3 and it started downloading.
But you have to pay for it with coins tho but the good thing is it costs 800 points, and that is the exact amount you can buy for 6 euro's or so. So basicly you are paying with points but buying them at the same time just as if you were paying with real money.
You can also buy the DLC from the BioWare site at http://social.bioware.com/page/me3-dlc. Unlike ME2, though, you cannot download it there — you have to get it via the same Information page for ME3 that Mr Healthy's answer mentions.