Are you having problem, like when you download a file from the internet or anywhere. And when you click on the file, they show you random numbers and letter you cannot understand. Then at the top it said cannot open in DOS mode. If that is the case, that is very common. I have also the same kind as problem with you and have try various methods but it did not work.
1) Make sure the file is zip file or other format but not exe. exe files are for windows and cannot open using mac. So when you open exe. files on mac, the file will show you a lot of random stuff like numbers and letters you cannot understand.
2) Try opening other zip files download from the internet and see whether you can open and view them without any problems if yes then the file you download that say cannot view in dos mode has a problem. It would be best to delete them.
3) Do a virus scan and check whether any files are corrupted. Use iantivurs. It is good and free. Download it and do a full scan.
Post back what you have seen when you follow my instructions. Basically there are no DOS mode on mac, so your problem is weird. Almost every mac users have the same problem around the world so you do not need to worry. And it won't harm your macbook at all also.