Yeah, ENMeval doesn't let you run ENMevaluate() with just one variable. I should probably fix this, but then again it is not advisable. Maxent is a machine-learning algorithm that excels at handling a bunch of variables and sorting through them to figure out which are best for your data (and this improves through tuning exercises). If you plan to use a single variable to make a super simple model, Maxent is probably not the best choice. In this case, I'd use a regular logistic regression model (GLM).
Also, if you want to get marginal response curves, running models with one variable is not how you do it. The point of the marginal response curve is that you have a model with multiple predictor variables and you want to see the marginal response of one of the variables. You get it by setting all the other variables to their means (or medians) and making predictions on a range of that one variable. This tells you how suitability is affected by that one variable when all the others are held constant.
ENMeval doesn't do this because dismo already does it pretty well.
Hope this makes sense.
Jamie Kass
JSPS Postdoctoral Researcher
Okinawa Institute of Science and Technology Graduate University