You can surely connect a midi keyboard/controller that’s ether 5-pin midi or USB midi class compliant, and it will work right away. I do precisely that (though with a midi guitar controller.) Just plug the USB cable in and voilà!
You can use the generators that exist in the Mod store. For a list of existing instruments, please see this list:
These are all plugins that take a midi input and generate sound. (I’ve chosen the “instrument” category, but you may also select “generators”.)
One of them, the DIE Fluid Synth is a device that plays samples in SF2 format – meaning you can load your instruments to it. Think of it as a classic sampler. We were already told that Sfizz may be coming to MOD as well.
What MOD doesn’t do is to allow you to load a VST instrument. They must be in LV2 format. I am hoping (more than believing) that Apple’s move to ARM processors will then make developers create ARM versions of their plugins. Also – and more importantly, MOD’s platform offers a way for developers to actually profit from their plugins, so it’s possible that one such platform would create an incentive for developers to undertake the work of porting their plugins to MOD.