there is check box called "Swap target model" at the last page of "import from data dictionary" wizard. If you select it your database will be the target model and you can get alter statements in DDL preview of merge dialog. It's the same with DDL script.
I've done this by following a tutorial and the Preview DDL window is BLANK. No matter how many times I try it's blank. The compare model window shows the changes but I can't get the DDL generated. Anyone else have this problem?
can you check:
1) about errors logged in log file - log directory of installation
2) in "compare models window" - whether objects are selected (checked)
3) there are changes that are not related too DDL generation