@Javelin
It is a separate paid extension...
Have you actually installed FredoSpline ? - either from its RBZ or using the ExtensionStore³ dialog auto-install...
It will appear under the ...Collection submenu.
BUT do you have the latest version of LibFredo6 installed and the SketchUcation toolset ?
Have you restarted SketchUp after the installation to sync things ?
Have you actually validated your downloaded license file using the License... dialog in the ...Collect > FredoSpline submenu ?
Doing so will also make its Toolbar available...