Hi @Fabster!
The missing dde here is not an R package, but instead part of Tcl/Tk, which is the non-R software library that R Commander uses to create its GUI. So basically, it looks like some of the non-R system components that this R Commander plugin expects to be present are missing on your system. However, I can’t really tell if that’s the whole problem based on the error messages above (the mentions of dde were only warnings).
If you're determined to get this plugin working, I'd start by reviewing the R Commander installation notes and the documentation for the plugin itself.
However, if you're only going through all of this in order to do a Fisher's LSD test, then you definitely have other options! 
There is more than one R package out there that implements a Fisher's LSD, but a commonly used one is agricolae. This page has a nice example showing how to use its LSD.test() function: R Companion: One-way Anova
Just a note, this is not the case. R Commander is compatible with RStudio (though you'd be forgiven for not realizing this, since it's stated in a footnote on page 25 of the Rcmdr introductory manual 
).