I understand your question in two different ways:
1. You want to choose a random file from a specified folder
2. You want to choose a random position in a single file in a folder.
I use randi here, but if you want to use randn you probably need to find a way to turn that into an integer. It's unusual to want to use a normal distribution for something like you seem to be describing though.
So...
If (1) Use the dir command to list files in that folder:
f = dir( myfolder );
ridx = randi(numel(f));
disp( ['Chosen file is: ' f(ridx).name] );
If (2) Open your file and determine its size, then seek to random position:
fid = fopen( myfile, 'rb' );
fseek(fid, 0, 'eof' );
fsize = ftell(fid);
rpos = randi(fsize)-1;
fseek(fid, rpos, 'bof');
...
fclose(fid);
