Click the Channels entry in FL Studio’s menu bar, select Add one… and then go to More…
In the dialog that now opens up, look for the list entry that contains the name of your u-he plug-in. If there is not one, please click Refresh - Fast Scan (recommended), which will instruct FL Studio to scan the plug-in directory for newly installed plug-ins.
If no new plug-ins get added to the list, then please make sure that you have installed your u-he plug-in to the correct folder. You can set the path for the external VST folder in FL Studio’s Options under File Settings - VST plugins extra search folder.
Once your u-he plug-in shows up in the list, make sure that a little black F in a box is displayed beside that entry. If there is no F but just a grey/white box, then click that grey/white box and an F with the black box will now be displayed. This informs FL Studio to “use this plug-in” and to show it in the plug-in menus.
You should now be able to load your u-he plug-in into FL Studio just like any other instrument or effect.