There are two features with import: test specification and requirements.

Test specification 

Testlink 1.7 supports XML import only. You can use a windows tool for conversion CSV format to XML (developed by Garyo Hosu). Download:

http://testlinkjp.org/modules/mydownloads/visit.php?cid=2&lid=10

The format of the CSV is as follows

TestcaseName,Summary,Steps,Expectedresults

CSV format is directly supported for versions 1.0.4 - 1.6.x (include also component, category and keywords)

Example of import test cases.

Requirements 

Requirements allow import two types of csv:

• simple   (consists just from title and content fields)
• Doors (accept CSV file exported from Doors software; include header)