If you have found a bug, you can report it using the Bug Tracking of the SourceForge site. There you can create a new entry by clicking 'Add new'. But before doing so, read the existing bug reports, maybe something similar is reported yet. In this case, add a comment to the existing entry.
Please only report something if you can provide precise and detailed information and have thoroughly verified the existence of the bug. You might take a look into SmillaEnlarger's Log tab and paste parts of the log into your comment if they seem helpful.