We currently have a loan on a retail shopping mall in which we have a deed of trust filed on the property. Our borrower leased a portion of the land to Whataburger, who built a building on the property that is mortgaged to us. We did not provide any funds to build this building. Who is required to enforce the flood insurance regulation, the lender of the real estate or the lender that provided the money to build the building (leasehold interest)?

Not you - you have no interest in the building - only the land:

Designated loan means a loan secured by a building or mobile home that is located or to be located in a special flood hazard area in which flood insurance is available under the Act.