You select a base mod (like Tacyns for example) - that's the one on the top. In your first screenshot it'd be Enemy Kingdoms, in the second it's Tacyns.
With this selected, you can add included mods. All included mods available are listed under available mods. As Tacyn wrote, you can select them with double click, what'll switch the mod to the selected mods list on the right.
So in your second screenshot, you could double click Enemy Kingdoms to add it to the right, you'd then have Tacyns base mod and enemy kingdoms active.
Basemods can't be included, that's why Tacys base mod (and maybe other mods) doesn't show up in the list of available mods, when you selected a other base mod.
So for your example: Tacy is a base mod - can't be combined with other base mods, but you can add included mods. Enemy Kingdoms is an included mod, you can select it on it's own or add it to other base mods.