The block you dragged in is from a different library which doesn't connect directly to Simscape Power Systems Specialized Technology. Looks like you're using an older release of MATLAB, but in newer versions they've tried to make this clearer.
What you'll have to do is create a custom resistor using a current sensor and a voltage source, so that you yourself implement the law V = I*R(t), where R(t) can be a time-varying input signal.
Note that you'll need to break the algebraic loop with some low-pass filter as shown below.
Refer to this documentation page for more details: https://www.mathworks.com/help/physmod/sps/powersys/ug/building-and-customizing-nonlinear-models.html
- Sebastian
