I haven't used the dreamcompute's keygeneration options (Though I'm 100% they work).
Let me tell you what I do.
First, get PuTTYgen. This is a key generator! Select the key size/type you want to use (I use SHA2-4096bit keys, but 2048 is the "standard" for these things). Click generate and move your mouse over the open space at the top of the window.
Now open up the dreamcompute panel and select "Access and security" then the "Keypairs tab", then click the "Import Key Pair", a window will pop up. Put any name you like (it's not important). Go back to the PuttyGen window, and highlight the key in it's entirety (make sure you get all of it) and paste it to the box labeled "public key". After click "Import Key pair" and wait for a success message. If you get a fail message you might have missed a line of the public key.
Now go back to the puttygen window. This is the most important step because if you do everything else right and mess this part up you will not be able to get back into your server.
Click the Save private key button. This is your secret key. I do not generally password protect my key, this means that anybody who can copy my keyfile from my desktop can use my key. You MUST encrypt your private key. You MUST PROTECT the key you just saved. An option for you could be Veracrypt ( I don't use windows so I can't vouch for it, but it is the popular option).
Ok so now dreamhost knows the public key we're going to use to launch our servers, we've got our key protected when we're not using it by encryption and that's awesome, now to getting the server running.
So I run some archaic distros usually so I will refer to your operating system as just *nix, at this stage there isn't much difference between the "flavors" if you will.
In the dreamcompute panel select "Images", then click on the "Public Images". Select your *nix and select "Launch". Choose the size of your instance and in the "Access and security" panel select the name of the key we imported earlier. Then choose your network and click launch.
Once your instance is running and you've attached a floating ip to it then open up PuTTy the client. I see you're using pageant and that's fine but let's make sure it works with PuTTy first. Input your address, go to authentication and add the key that I stressed should only be decrypted while in use. Then click connect and use dhc-user as the login name. Let me know if this works for you. (Note: After you're authenticated with the server you may re-encrypt the key, it only needs to be decrypted while you're authenticating.)
I hope this works, I don't think I've missed out on anything, cheers!
Edit: Just saw your edit. That is indeed the problem. An instance is what you're trying to log into, no it cannot be changed once it's launched you'll have to relaunch, I recommend using my instructions because I think you've actually ruined your keyfile with the conversion you attempted.