Thanks Markus, I didn't try it but I will!
If the package is anything like the one in VisualWorks (In other words, LogNormalDistribution a subclass of ProbabilityDensityWithUnknownDistribution), you probably want to implement
LogNormalDistribution >> distributionValue: aNumber
"Answers the probability of observing a random variable distributed according to
the receiver with a value lower than or equal to aNumber."
^aNumber > 0
ifFalse: [0]
ifTrue: [normalDistribution distributionValue: aNumber ln]
rather than rely on the supers implementation, if you plan on using it.
Cheers,
Henry