If eWallet says that a wallet file is in use or read-only, then it isn't able to write to the file. Here are the reasons why:
Your wallet file is already open in another instance of eWallet. Try closing all eWallet windows and try opening the file again.
Your wallet file is actually set to be read-only. To fix this, right-click on the file using Windows Explorer, then pick Properties and un-check the Read Only box.
Your wallet file is located in a folder that your user account does not have write access to. Try moving your wallet file to your My Documents (or just Documents in Vista) and see if you can open it from there.
You are opening your wallet from a write-protected USB drive, memory card, or a read-only CD.