In Mac OS X 10.10 ('Yosemite') only:
The speed sliders in EcoBeaker SVL modules are missing a background and can be hard to see. Instead of a round control on a horizontal "slider track", the background track appears to be missing and the control is a small gray dot.
This problem is inherent in the native user interface for Yosemite; there is no workaround as such for the problem seen in the EcoBeaker SVL modules. Note that the slider-dot control functionality is still intact --- users can adjust the speed of the simulation by moving the control back and forth, as the instructions direct you to do.