I have the same issue. If I enter my Electrum seed into Electrum Cash, the "Next" button is greyed out. I've enter the seed in vertically, horizontally, with spaces, with a comma. Nothing works 
The correct way is to just put a space between each word. "word1 word2 word3...";If it doesn't work, maybe the seed you are trying to enter is wrong? Go back to your Electrum wallet, then go to Wallet -> Seed and recheck your seed. If for some reason you still can't continue, you can just open your already existent Electrum wallet file into Electron Cash as they are both compatible.[/list]