You'll have to :CC2EXPLODE: the symbol, that gives you access to the individual parts of it.

I have a blog article about X'es that might help you:

You're using the wrong version of the installer. The one you are using is the one for CC3, not for CC3+ which is what you have. You'll find the right one on your registered user download pages.

Not sure why you're referencing DRM here though, a technical issue like this doesn't have anything to do with DRM. This is just a compatibility issue where CC3 add-ons can't be used with CC3+ and vice versa, not a protection scheme.

I have a couple of videos on creating symbol catalogs which may help:

vthumb(dRRa6ctWPJY|Creating Symbols - Part 1 - Finding and Preparing Images)

vthumb(cYxAIPa7dak|Creating Symbols - Part 2 - Creating a Symbol Catalog)

Doing a map of that size isn't a problem in CC3+. But it will be a bit time consuming, a map like that is not done in an afternoon.

Since you're mentioning the isometric Schley city set, i am not convinced it will be appropriate for such a large city. I haven't tried, but the isometric sets are generally better when you can see the details a bit more clearly. Might be a good resource for mapping closeup of individual districts for example.

Personally, I am not convinced it is a good starting point doing such a large map if you are unfamiliar with CC3+ though. I recommend starting with something simpler to get some experience first, or it is likely to be overwhelming.

You might want to have a look at this live stream recording where Ralf starts making a small city. Far from the size of your desired city, but shows off the tools and techniques.