Importance of grades when applying to Master's degree program?
Actually, admissions to master's degree programs are quite different than PhD admissions. Doctoral programs are research programs, and therefore promise as a researcher is perhaps the most important criterion of all to satisfy.

However, at the master's level, this may or may not be the case—many master's programs feature no research (purely coursework), or only a small amount of research (for instance, a one-semester research project, or a lab course, or something similar). In such cases, research is not emphasized nearly as much, and undergraduate track record takes on greater significance. Similarly, "terminal master's" programs—ones that represent "end degrees" in their own right—will likely have less of a research background focus than degrees that are intended to prepare a student for doctoral studies. 

The other issue is the style of admissions: for "US-style" admissions, research will likely be more important than it is in "European-style" admissions, where research plays essentially no role at all in admissions to master's programs (although research is a prerequisite for PhD studies!).