Hello afrec,
There could be several reasons why the imported customers can not be found. There are two checks I do with customers when they can't find customers. First, check the filters on your Customer and Contacts List grid. The customers imported may be checked as inactive. Second, export the Customer table. In the same Integration Tools screen, select Export a table to a field and the Customer table. Click Export to file... to get a .csv file with all the customers in the table. You can also check the contacts and leads table.
When you did the import, did the progress bar complete successfully and did it say it was successful? Was there any time-outs or error messages?
I hope this helps,
Greg