Can I suggest you contact a local friend who is familiar with Linux. The fact that FSL can not open the file suggests the problem is not with MRIcron, but I am not even sure you are in the correct folder. You would want to run "ls -l" from the command line to make sure your terminal is in the correct folder and that you have permissions. Someone familiar with computers can help you out. The Unix handout here might be a good starting point.
https://fsl.fmrib.ox.ac.uk/fslcourse/