You have to:
SDL Trados Studio 2017
1. Sign out - go to the Language Cloud account button in the top right corner of SDL Trados Studio

2. Go to the
Sign In button

3. Choose
Sign in with API Key option

4. Use the API Key to sign in
SDL Trados Studio 2015
1. Go to Project Settings
2. Language Pairs
3. All Language Pairs
4. Translation Memory and Automated Translation
5. Add - SDL Language Cloud
OR
Select SDL Language Cloud - Settings
