FAQ: Why can't I import my exported (.txt) eWallet wallet file?
eWallet's export format is designed to provide you with a flat list of all of the cards from your wallet file. The data isn't formatted in any way, although it is easily readable by a human. While you can use an exported text file to print out your wallet to keep as a hard copy, eWallet cannot import an exported eWallet file.
If you want to make a backup of your wallet file, either make a copy of the .WLT file yourself, or use the Save As...eWallet File menu option (bubble icon in upper left corner) to save a copy wherever you want to keep it.