Before importing leads from Salesforce to your campaigns or list, you need to first connect Salesforce to Instantly and then create a "List view" in your Salesforce, from which you will import the leads.
Instantly is fetching "contacts" data from Salesforce. If you'd like to import "leads" from Salesforce to Instantly, you'd need to convert them into "contacts". You can follow this instruction to convert them: https://help.salesforce.com/s/articleView?id=000388015&type=1
Once your list is created, follow these steps to import leads to your Campaign or List from Salesforce:
Step 1 - Click on "Add Leads".
Step 2 - Select Salesforce:
Step 3 - Choose a Contact filter (to import contacts with a specific Salesforce list) and click Import:
Step 4 - Map the corresponding fields and click "Upload all".
Instantly will check for any blocked contacts and duplicates across all campaigns when importing leads.