Your PayPal Merchant ID is the "ID/Account Number" associated with your PayPal account.
Within your PayPal account, you should be able to locate your Merchant ID in your account settings by clicking the wheel at the top right, going to the Business information section.
In LT Give, your PayPal Merchant ID is listed in Settings > Manage PayPal Settings once your PayPal account is linked to your LT Give account.