So looks that everything is running smoothly with no additional lubrication. The problem is in project structure. Scala IDE works with Eclipse (debug perspective strictly speaking) and because of it there are some limitations. First of all looks like folder structure should reflects package structure to allow debug machinery to find appropriate file (more accurate is to say about compilation unit I guess). In this particular case
src/Mini.scala should be moved to src/mini/Mini.scala
debug/Mini.scala should be moved to debug/mini/Mini.scala
I'm aware that there is not a need to reflect packages by folder structure in Scala but Debug Perspective was done for Java.
I would be grateful if you could confirm it.
W.