Who is responsible for air conditioning equipment & venting in a commercial lease, tenant or landlord?

It depends on your lease. Typically major capital improvements (like those in your post) are the responsibility of the landlord but leases vary. Refer to yours. If you still have questions, consult with a real estate or contract lawyer for a review of your options.

Mr. Massey hit it right on the head. Your lease should speak directly to this point. Chances are it is the LL's responsibility unless you contracted otherwise. Did you have someone review the lease before you signed it?

I agree with the previous answers but don't be surprised if the lease actually makes you responsible for the AC repairs. I have seen a number of such provisions in strip mall leases, particularly if each tenant space has its own stand-alone AC. If a review of the lease indicates that you assumed responsibility, be aware that it may also require you to use a repair service approved by the Landlord and there may be specific rules regarding access to the roof and how repairs and roof penetration are to be handled.

The lease must be reviewed to determine the scope of the tenant's and landlord's responsibility. You would benefit by having an attorney whose scope of practice includes commercial real estate review the lease and advise of your rights and options.