In 3 simple steps

1. Go to Confluence administration and find menu item "Logging and Profiling". The direct link would be something like https://YOURSERVER/admin/viewlog4j.action 

2. Add the following entry
   

   com.vertuna

3. ConfiForms is using the same log used by your Confluence. ConfiForms log entries are from 

   com.vertuna.confluence.plugins.confiforms.util.ConfiLogger