Manually posting a payment and/or adjustment is done in the Claims Management Widget on the Outstanding Tab by patient, by claim.

Claims Management Widget Outstanding Tab

Search to find patient
Select the Gear > Revenue Details to open the Revenue Details Float to enter a payment and/or adjustment

Selecting Manually, a Revenue Item float will open to manually select what revenue line to post against

Selecting Proportionally, PIM will automatically post the payment proportionally to the revenue line