@eklein12, welcome to EnergyPlus, OpenStudio, and UnmetHours. It's been quite a week for you! You hit it, OpenStudio does not have native models for all EnergyPlus objects although coverage is getting better with every release. New objects usually appear in the OpenStudio SDK and API first so that you can access them programmatically, then make it into the graphical application shortly thereafter. There is no way to "download" missing features into the OpenStudio application so that you can manipulate them graphically, but it is possible to access them via "measures", scripts that transform models in specific parameterized ways. "Measures" are a great way to extend the functionality of OpenStudio either as a stop-gap—until the missing feature is implemented natively—or even permanently—because the feature makes more sense as a script.

Right now, it looks like desiccant systems are not available via a measure (https://bcl.nrel.gov/search/site?f[0]=imfieldmeasure_tags%3A956), but maybe someone reading this question will be motivated to write one!