During the Great Depression, how did the Stock Market Crash of 1929 lead to companies losing money and resulting unemployment?

As historian David Kennedy has observed, "the Crash in itself had little direct or immediate economic effect on the typical American." This is because, according to official estimates, only a tiny fraction of Americans, less than three percent, actually owned stock when the crash occurred. Indeed, Kennedy, whose book Freedom From Fear is among the most respected syntheses of the period, observes that

...most responsible students of the events of 1929 have been unable to...

As historian David Kennedy has observed, "the Crash in itself had little direct or immediate economic effect on the typical American." This is because, according to official estimates, only a tiny fraction of Americans, less than three percent, actually owned stock when the crash occurred. Indeed, Kennedy, whose book Freedom From Fear is among the most respected syntheses of the period, observes that

...most responsible students of the events of 1929 have been unable to demonstrate an appreciable cause-and-effect linkage between the Crash and the Depression.

For many historians, the Crash was more of a symptom of the economic difficulties that were beginning to plague the nation. It reflected an awareness among investors that stock prices had become, in effect, a speculative bubble, disconnected from the actual economy. In fact, even after the Crash, many policymakers and economists believed the economy was only experiencing a temporary downturn. Unemployment figures were rising, and some banks were failing, but the bottom had not yet fallen out, even by late 1930. 

Most historians agree, however, that even though the stock market crash did not cause the Great Depression, it did help hasten the onset of the economic malaise that gripped the country and the world during the 1930s. This was especially true in the banking sector. Many banks had invested heavily in the stock market during the boom years of the 1920s, and when the market crashed, they lost billions. This led some to fail, which contributed to a credit squeeze and eventually bank panics, as investors rushed to get their money out of presumably undercapitalized banks. These bank panics, which reached their peak in 1932-33, brought the Depression home to millions of frightened people who saw their savings wiped out.

The Crash also helped contribute to the atmosphere of fear that contributed to the Depression. This, in fact, was probably its most direct effect, and the clearest possible answer to your question. It permanently destroyed the aura of optimism of the 1920s. People began, either through prudence or necessity, to lessen spending on the consumer goods that helped drive the economic growth of the previous decade. Businesses that manufactured these goods (especially the automobile) had to cut back on production, and then on wages and jobs. This is what actually led to rising unemployment rates. As mentioned above, they took their money out of banks, and they quit borrowing money. Banks, similarly frightened, began to call in loans. With less money circulating through the American economy due to tightened credit and rising unemployment rates, the economic downturn that accompanied the Crash became a tailspin.