Loans with availability are listed under Available Loans. You need to have funds in your account in order to purchase loans on the Secondary Market.
Click on the loan you wish to lend in, enter the amount in the box and click the I am not a robot box. The system will then decide whether a captcha needs to be solved, solve the captcha and a green tick will appear in the robot box, click to invest and then click to confirm your lending. The system will then confirm the lending providing no another lender has bought the sum available in the meantime.