SOURCE


1. Download steam, register an account
2. Download vindictus, register your nexon id (the one you use for Maplestory) with steam
3. add some money to your steam wallet
4. Launch vindictus
5. Click the buy premium slot button
6. Click recharge nx
7. Buy nx cash through steams in app purchase system

It gets applied to your NexonID, which means you can use the same NX in Maple :D

Remember to always test first with small amounts (like 5$, which is the minimum) before doing large purchases.