To obtain the Atterax you have two options, being the first one to buy it through the Market Console or directly from the Arsenal of your ship. It will cost you 175 Platinum, but if you choose this option you also receive a weapon slot in your inventory and an Orokin Catalyst already installed.
The second option is to buy the Blueprint in the Market Console and craft it in the Foundry of your ship. To do that you need the required resources. Remember that this process takes time and effort.