Without getting on the topic of M$ is bad or Excel shud only b a spreadsheet (though I wud like a lot to discuss on this

), here is the way to do it if you have your test cases already written in Excel:
1) Copy your excel to a new file (so that you have a backup)
2) Remove all your columns and keep only the columns corresponding to Component, Category, Summary, Steps & Expected results of TestLink.
3) Save this file as CSV file.
4) Import this file into TestLink.
This could cause you problems if you have carriage returns in the text in your excel cells. If you have such cells (text with carriage returns) dont bother, just copy your test cases manually.

Sadly, this is how we had to move nearly 4500 test cases from Excel to TestLink. But now that it is done, we are very happy
