I am coding with computercraft and I do not know where to post the questions - Arqade or Stack Overflow? Or both? If so how?
You should ask your ComputerCraft programming questions to StackOverflow with tag computercraft.
You can ask non-programming questions like (not sure what I can include, I didn't use ComputerCraft for years) crafting recipes of computers that come with ComputerCraft.
Please keep in mind that you are likely to get downvoted if you ask that question as it is a low effort question (can be found on mod's forum thread etc etc), however I hope that you get the base idea of "gameplay related parts to Arqade, code related parts to StackOverflow").
1I just saw that you have a question about ComputerCraft with that tag on SO already. Let me clarify a bit: Your question was downvoted due to lack of detail (in this case, probably because you didn't include sample code), try reading the How do I ask a good question? article of StackOverflow.– aveAug 27, 2016 at 19:49