The first error message tells you precisely why it's happening -- the third line of the file doesn't have the same number of values/delimiters as do the previous two lines. load -ascii can only handle regular files.
Whoever named the file with a .mat extension erred in doing so as it leads to expectations of what file content will be that aren't met with this particular file.
We would have to see the file to be able to have any more suggestions regarding how to read it specifically; in general there is textscan for fscanf which can read most any file structure but have to be coached on how to do so if the data isn't totally regular in pattern.
