This is one of the fun, annoying quirks of the API.
Currently there is no pragmatic way to find a user based on their name instead of ID. Furthermore, there is a better chance that Jon Skeet will be banned from the site for misuse than the dev's deciding to reveal people's OpenID.
For now, you should direct users to go to their profile page and look at the URL, look for the numbers, and know that is their user id. The sooner they know it, the easier it will be for them to interact with all these applications.
I can imagine this problem will be slightly lessened by the 2nd version of the API when you are able to use authentication, as you will probably know the ID of the currently signed in user without them having to enter it.