How can I make up for weak grades while applying for a masters?
I would look for a purely taught program (i.e., no research component) terminal masters program and demonstrate that you can, and expect to, self fund. The expectations of self funded students on terminal masters programs are very different from masters programs designed to feed into PhD programs. I would also target smaller programs where applications are looked at more throughly. The US system relies on standardized tests. Take the GRE and a subject test and do well (really well). Finally, think about why you did poorly in you undergraduate studies and what has changed since then. Then figure out how to express that in your statement of purpose.