Your flickr username or flickr id isn't actually the username you login with but rather a unique ID generated by Flickr. Its a slightly strange set of numbers at letter with an '@' symbol towards the end. For example 12341234@N12.
We've added the ablity for users to use their flick id on iMapFlickr just for the nerd factor, but most normal folks will be just as happy entering their email address or their flickr url.
Finding it can sometimes be tricky, often its visible in the URL (take a look at the pic above) you see when you are browsing your photos, or there are numerous online tools that find it. Some useful online tools that will get your flickr id are:
We use your flickr id internally to search your flickr photosets, but you don't need to know it - just use your email address or photostream URL instead (we do!)