Hello Christos,
In order to add the Infragistics components inside the toolbox of the designer in Visual Studio, there are two approaches.
The first approach is to create a tab inside the Visual Studio toolbox, this could be done by right clicking inside the toolbox and pressing “Add Tab”. Then to add the controls inside it, right click on the tab and select “Choose Items…”. Once the dialog opens up you should be present with a list containing controls, select the check boxes corresponding to the Ultimate UI for Windows Forms controls and components that you want to add to the toolbox.
The second approach is to create a tab and add the DLLs you wish to include inside the toolbox by copying and pasting them to the newly created tab. The assemblies that contain the DLLs could be found under this directory:
C:\Program Files (x86)\Infragistics\2020.2\Windows Forms\Bin
Below you may find a link that describes both methods stated above in further detail:
https://www.infragistics.com/help/winforms/introduction-adding-controls-to-the-toolbox
Please let me know if you have any questions.
Regards,
Ivan Kitanov