Hi JP,
I was just about to post this but saw you figured it out already. I'll keep my reply in tact below though.
The first thing to check is whether you also reference our Grids and Shared assemblies. Shell controls inherit TreeListView/TreeListBox that are defined in Grids. Maybe if you don't have the Grids reference, the VS designer doesn't realize the shell controls are actually controls, and it therefore reports that error?
If you have those Grids/Shared references too, it should be working fine if you are absolutely positive that ALL your Actipro assemblies in your bin folder are the same exact version/build numbers. You might want to walk through there and double check the versions of the assemblies VS copied there. In v20.1 and later, we started offering NuGet packages on nuget.org, so it makes consuming proper assembly versions easier.