If you are developing using QA Build, at those points in the development of an application when you need to create or edit a map, QA Build automatically starts the Map Editor and opens the appropriate map. You can also start it yourself by double clicking its icon in the DP4 program group or in the DP4 main menu, or running MAPEDIT from a command prompt.
When you start MAPEDIT like this you you have to open a database, before any options become available. Then you will usually want to open a mapset, because even if you do not want to edit a map, opening the a mapset is a good way to discover maps you want to delete, copy (or copy over) etc.
If you are used to the way other DP4 utilities work you may find MAPEDIT takes a little getting used to. Another utility, MAPMAKE, is available as a front end into MAPEDIT, and works more like other DP4 utilities. You may find MAPMAKE makes it easier to select the map(s) you want to work on. MAPMAKE also contains facilities to print out maps or to create MAPPOST format files so you can distribute changes to maps, and has some specail facilities that are useful if you need to work on the same map in more than one language.