How to get into build mode in The Sims 4 Demo

Are you referring to the Sims 4 CAS demo? I don't think that the Sims 4 Demo has been released and will be released to the public.

In the CAS demo - it is plainly just Create-a-Sim (CAS) which means that you cannot venture outside or into the actual game. There is nothing else to it. 

I hope that answers your question?

Yeah, I downloaded the demo too, not the 'Create a Sim' demo but the other free 48 hour play demo. Cannot get to build mode so if you want to just create sims for 2 days and not be able to save, this demo was a waste of time as it took my slow ISP over 24 hours to download. I'm so disappointed that I can pay with building which is the reason I have played Sims and Sims 2 for years. 

The 24 game time should be the whole game.

What happened if you clicked ok on the Sim, did it just save the sim ?

No, it said there was no save option. It's so annoying, it took over 24 hours to download the demo because my connection is so slow and to not be able to experience the build function, which is what I wanted to see is a bummer.

There was no check mark in the lower right corner ?

If you choose the check mark it advises you there is NO save option. The game opens to a plain CAS screen, you can create sims but you cannot save and you cannot get out of the CAS screen to go to neighborhoods. 

And this is what you've downloaded ?

https://www.origin.com/en-us/store/free-games/game-time ?

Sorry, but I'm super confused. Game time should be the whole game (never tried it myself though)

EDIT: Do you see two tiles in your Origin ? One for the CAS Demo and one for the full game (time limited) ?

There was a choice of download, the Create a Sim and the full DEMO. I dl'd the full one.  I'm bummed.

And you didn't play it last time it was available ?