I have an account I have opted in. I only have the private key of the Symbol account, not the mnemonic. This is sufficient, isn’t it? I ask because the process mentions to keep the mnemonic carefully, which I didn’t for this account 
I can move the XEMs if needed
thanks
If you have a private key related to account (pub key) you opted in then all is ok.
In wallet, you need first create profile and inside it will be an option to import a private key.
Check your opt-in status:
http://report.experimental.symboldev.network/
http://explorer.experimental.symboldev.network/