The RandomPlus thread contains posts that explain how the plugin is supposed to work/be used.
The first post explains that the plugin gives pretty much a modification of the random() expression.
This system expression expects a range (a minimum number and a maximum).
The action "RandPlus" adds a seed to it.
Just go through the two pages of the thread, it's explained.
Moreover, you can check the inline help for the plugin (from within Construct2 itself) to see what values are expected in what field. Like for any other plugin.
.count and .pickedcount are common expressions to all plugins.
As for Rex's generator, you first have to use the Seed() action to set your seed, and then get the random() expression using that seed.