I would call this a practical joke. There is a leading space in the names of the datasets. (Caveat: This is the first time I encounter the CFD General Notation System.)
You didn't tell what you tried to read, however, try
>> num = h5read( 'h:\m\cssm\xy.h5', '/ format' )'
num =
73 69 69 69 95 76 73 84 84 76 69 95 51 50 0
and
>> num = h5read( 'h:\m\cssm\xy.h5', '/xyplane/sidesplane/ZoneType/ data' )'
num =
85 110 115 116 114 117 99 116 117 114 101 100
This is an excerpt of a h5disp output.
