For Linux, there is a gnu utility called md5sum that will calculate a checksum for a given file with (see the online manual for full description):
md5sum <filename>
Under UNIX the md5 utility will do the same thing with:
md5 <filename>
The MD5 checksum you provide should look something like this:
d41d8cd98f00b204e9800998ecf8427e
Don't paste in any surrounding text -- just enter the plain checksum.
For Windows (95, 98, NT) the md5sum utility is available as part of the cygwin distribution.