As a pre-requisite you need to install the latest version of the 3CX Assistant for Version 9.
Download the Salesforce plugin package from here
Step1. Install 3CX Assistant Version 9 with at least 1 CRM option for outlook enabled.
Step2. Start the Salesforce installation.
Step3. After setup is complete, start the 3CXAssistant and go to File/CRM settings.
Step4. Click the CRM Plug-ins tab and check “Enable Salesforce Plug-in”
Step5. Enter your login details for your online Salesforce account and save. The plug in will register with Salesforce online.