I recently did this. To be honest, LinkedIn's Marketing Solutions platform leaves a lot to be desired, and it isn't super intutitive:
Follow the instructions for Adding LinkedIn as a LaunchPoint Service, and go to the 'Tools' tab within the LinkedIn Campaign Manager and click 'Matched Audiences': (I'm not sure if non-admins can see this - let me know!)
Go to the 'Account lists audiences' tab and click 'Connect to marketing automation'
What, here, is labeled 'Public Key' corresponds to 'Marketing Solutions API Key' and the 'Secret Key' is the 'Marketing Solutions API Secret'.
Then, follow the instructions to Use a Marketo List or Smart List as a LinkedIn Audience Segment. From the LinkedIn side of things, it may take up to 48 hours for your Audience Segment to appear.