If you've added your interface either in Audio > Settings or as a source in your source pane, it should appear in the Mixer pane, with moving VU meters to indicate volume.
Underneath the meter, from left to right, is a volume slider, a mute button, and a gear icon.
Click the gear icon and from the drop-down menu that opens, choose Filters.
A new window will open named "Filters for <device name>". The left pane will be a list of currently applied filters (empty if you haven't added any yet). At the bottom are buttons to add and subtract a filter, and arrows to move selected filters up and down in the order. (Filters are applied to the source according to their order in the list, top to bottom).
Click the "+" icon, and from the drop-down menu that opens, choose the item "VST 2.x Plug-in. You will be prompted to give this filter a friendly name so you can remember what it is doing, and then a window will open with a drop-down menu that lists all the installed VST 2.x plugins OBS could find on your system, a button that says "Open Plug-in Interface" so you can configure the plugin, and a checkbox to always keep the plugin UI open when active.
If you don't see the plugin you want on the list, make sure it is installed where OBS looks for it:
When you are done, close the Filters window. Filters in the filter pane also have a show/hide icon on the left like sources (eyeball icon) so that you can toggle their effects.