You can add notes after an offline interaction with your lead by navigating to to the Lead Details page. Navigate to Dashboard > Leads > Manage Leads. Click on the desired lead. You will be redirected to the Lead Details page. Here, you can record the offline interaction.
Click on Add Notable Activity. Select a suitable activity from the drop down menu. Select the date and time. Enter relevant notes and add the activity.
You can now see the activity in the activity history.