I've been using 1Password for years on my iMac, and normally am asked for my master password when the application opens.
Today when launching the app, it says ""Welcome to 1Password, to start we need to setup your data file. If you are new to 1Password, click New Data File, otherwise click Find Existing".
I've never seen this before --- and do not know how to find this data file.
1Password Version: 3 for Mac
Extension Version: Not Provided
OS Version: OS X Yosemite 10.10.5
Sync Type: not sure sorry