Currently, the simplest way to upload multiple products is to use the Easy Populate module.
For Zencart version 1.3.8a, you need to fetch, extract and upload Easy Populate version 1.2.5.4 to your server. This is available here:
http://www.modhole.com/modhole/index.php?module=documents&JAS_DocumentManager_op=viewDocument&JAS_Document_id=2&4b585d2fbbc5810fc7dc00998f1c2f31=379b4767004e7d8f298ac8317472edc5
A version called 1.2.5.5 is available from the Free Software Addons of this site, but some people report problems with 1.2.5.5.
1.2.5.4 from modhole uses spreadsheets (typically MS Excel) to manage the data, after the spreadsheet is saved as a tab-delimited TXT file.
A ZC forum member called phazei has recently developed a CSV version, and there is a support thread on the forum. Please note that phaezi's version requires PHP 5.2 installed on your server, in order to work properly.
forum member schoolboy has written a comprehensive TUTORIAL which you can read in the WIKI (Search for Easy Populate in the WIKI).