2023 Update
See the community FAQ:
FAQ: Creating Virtual Devices with the new architecture (Without the IDE) (2023)
original post
Not sure why you can’t get into the IDE, that should still be working for now, although it will go away in the next development phase. 
Are you using the universal sign on link? It’s working fine for me, but of course you might be on a different shard.
https://account.smartthings.com
meanwhile, there are two different methods you can use without the IDE.
- if you have an android device, use the labs feature inside the smartthings app and there is a virtual switch creator there. But it is not available for iOS.
- alternatively, if you want to go ahead and get started with edge, which is part of the new platform, there is a community created edge Driver that lets you create virtual devices. (You do have to have a smartthings hub to use edge drivers. That was not a requirement for the IDE, so I just wanted to mention it.) Edge itself is still in beta, so all edge drivers are currently in beta, but this one seems to be working quite well and a number of people are trying it.
First, the FAQ on what edge is if you’re not familiar with it (The topic title is a clickable link)
And here is the link to the Edge virtual device generator:
[ST Edge] vEdge Creator: a virtual device generator for end users