Forge for Minecraft 1.8.1 says I have to run MC 1.8.1 manually first (which I've already done)
There is not a version of forge for 1.8.1 by the looks of it.
Run the 1.8 (not 1.8.1) of Minecraft and it should work.
Go into the Minecraft Launcher. Edit your profile and look for [Version Selection] > Use version: And choose for version 1.8. Let it download by clicking play. Close the game and re-run the Forge Installer. It should work now.