Consider reading this short piece by Rothbard entittled "Professor Hebert on Entrepreneurship."
It seems to me that we can dissolve Rothbard's critique by a simple terminological change. Let's call an entrepreneur "alert" if he is perceiving a true or genuine profitable opportunity and "deceived" by a merely apparent but in fact false and loss-causing profit opportunity. It's true that it is not known in advance whether he will profit or not, because the future is uncertain; the whole purpose of being alert is to spot truly profitable ventures better than others -- but that is beside the point: an investment into a production process will either be profitable, break even, or be unprofitable; there is no fourth possibility. We can judge whether an entrepreneur was alert only after the results of his actions come in, but: a given production plan will objectively either work or fail to work; it's just that it is revealed as such only once the goods are sold to their consumers. And uncertainty does not necessarily entail unpredictability; for example, an omniscient God can predict our entrepreneur's status.
We can modify Kirzner's example to allay Rothbard's concerns as follows: as he is walking down the street an "alert" man will grab the $10; a "dull" man will miss the $10, and a "deceived" man will fall into an open manhole. And you can either profit if you are alert; sell your labor and spend your money, thereby breaking even if you are dull; or lose your capital if you are deceived.
Of course, any entrepreneur must risk capital in putting his idea into reality. But, again, if by alertness we mean foreseeing the future correctly, then the idea man's actions, when he receives the starting capital, will by definition succeed.
Objection: knowledge can exist without power to put it into reality. Again we can redefine alertness slightly as noticing only those opportunities of which one is capable of taking advantage. An untaken opportunity is a purely speculative creature, and economics, after all, deals not with contemplating men but with acting men.