There are probably ways to break/remove the password protection on START and SPELLCASTING pages, but I don't know how to do that, and I do not have the passwords. Somebody mentioned how to make the TABLES page visible, but you will have to search these forums for that, and it is really just a bunch of tables with no documentation attached.
It would take some real excel expertise to add new classes (as opposed to sub-classes), so adding new classes such as the Blood Hunter, Mystic or Artificer would be a serious set of challenges. There are other parts that are hard coded and do not meet the needs of some of the newer official materials (for example, only two domain/expansion spells per spell level). Other parts can be "hacked" in with a combination of using the CUSTOM page and some excel formulae-kungfu.
If you can not make this generator fit your needs, you may want to look into the free program Aurora Builder, which has basic 5th edition support and a community supported extension for what they can not legally support. A Non-free option that I like is Hero Lab, which also provides basic 5th edition support and a community project to support the rest. Neither are as "clean" as this generator, in my opinion. Of course, you can also use DDB if you don't mind their limitations (subscription preferred, buying sourcebooks [again], tied to internet access, etc).
The WotC/Hasbro lawyers have struck hard, and the wound is grevious.