P.S. the others have helped you open it. But I'll help you one step more. If the tweak doesnt mention this.

MSDOS.SYS is a read only file, so before you open it with notepad or whatever, right click on the file and remove the checkmark next to read only and click apply.

Make sure to put the checkmark back after you add the tweak!.

Also do not delete any existing part of it!

If you get an "Error in MSDOS.SYS" when you boot then you should check your syntax.

Most of the MSDOS tweaks are either going to turn something off (0-zero) or turn something on (1-one).

IE to turn off the annoying windows logo you would add

logo=0

You can also add lines to disable drivespace and doublespace disk compression support. Which for some bizarre reason is enabled by default in win98. But then win98 installs the fat16 to fat32 converter even if you enable large drive support.

Lots of MSDOS tweaks. Be sure you understand their effects fully before applying them as you could make your system unbootable.