You must be working from a Windows machine. Helvetica is not provided on Windows computers, and isn't licensed by any Windows program I know of for use. You'll either have to buy it for your use or use a fake knockoff version. I've seen a few fake Helveticas for Windows and they don't work well at all.
Using it in your font stacks is fine, as long as you provide a fallback:
And definitely put Helvetica in front of Arial.