The Level Builder is the main feature of Mega Man Maker. You can build levels with 1000 screens to work with (50 horizontal screens by 20 vertical screens), select from 111 Enemies, 42 Level Objects, 28 Bosses, 78 Weapons, 123 Backgrounds, 187 Tilesets, 3 playable Characters, and all 63 music tracks from the original Mega Man games plus a few added by the developers. All of these fields are constantly being updated, with new entries being added.
You can change the color scheme of the builder to Blue, Red, Pink, Purple, Yellow, Orange, Green, Grey, White, and Black. You can share your levels by going Online and selecting Upload. There are thousands of levels online. Once uploaded, you might see your level there too!
The builder is introduced in the Tutorial on how to use the builder. It works in seperated Screens, where tiles and Assets can be placed. There are over a thousand screens, all of which can be seen on the Map and altered via putting in assets.
.MMLV is the format used to store data about a Mega Man Maker level. This format can be edited with text editing programs, such as Notepad. Editing the text in these files can cause some bizarre glitches, and can change stuff, such as making a background be only inside a tile part (used in the original series in stages like Bomb Man's) and disconnecting tiles, or even doing a Mega Man sprite as a block.
Accessing .MMLV Files
In your directory, type %localappdata%/MegaMaker/Levels. This will bring you to all the levels you downloaded, and made. Every change you make to these files will affect said level.