[MEW ➡ Mist] Access your MyEtherWallet account in Geth / Mist
Mist & Geth
Mist files are typically found in the file locations below, but it's much easier to open Mist, select "Accounts" in the top bar, select "Backup", and select "Accounts". This will open the folder where your files are stored.
Wherever you saved it. ;)
It also was emailed to you, so your email inbox and any older email addresses you may have had.
Search for the file called
"ethereum_wallet_backup.json". This wallet file will be encrypted with a password that you created during the purchase of the pre-sale.
Hopefully you find it and can access it.