It seems that a lot of people here want to deny the property holder the right to manage his own property. Whether a business owns or is leasing the land/facility, they are legally in control of pretty much every aspect of that property.
At the very least, if the person in possession of a property instructs that "no armed person may come beyond this point" it makes you an illegal trespasser and subject to arrest and prosecution under most interpretation if you enter the property armed.
Those rights extend all the way to the front gates of the parking lot. You have a declaration of policy that you cannot come on company grounds carrying gun/liquor/drugs, etc, the property holder has every right to dismiss you from your job, escort you bodily off of the premises, and have you arrested for having come on the property armed after being instructed not to.
Legal environment may vary, and prosecutors will choose whether to enforce, but that's the way the situation stands. Your poorly defined second amendment rights do not trump property rights set down in thousands of years of common law and US, state and local codes. If not for a slight alteration of the wording, your declaration of independence would have said "life, liberty, and property" instead of "happiness." The idea at the time was far more focused on freedom of the individual rather than on individual responsibility to respect the rights of other individuals.