The sessions directory does not exist: /tmp. ??

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http://forums.oscommerce.com/viewtopic.php?p=228131#228131
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Warning: The sessions directory does not exist: /tmp. Sessions will not work until this directory is created.

The osCommerce store front is working well on my Apache server, MySQL database imported etc.. but I get the above error when making any product choices.

I've created empty directories called tmp inside every directory of the store but it still says /tmp does not exist.

Where is the directory called tmp supposed to be created? Why wasn't it included in the store download files?

Thanks,
MB