Hello Michel Dekock,
Could not find your FR2500 (but close)...
Has worked every time for me...
Mount the paper roll on the wire bracket... feeding
over the top of the ROLL (TOWARD THE MACHINE).
With the keyboard FACING away from yourself
Fold the FRONT EDGE of the PAPER on to the
side to form a chisel point... and insert the point
(folded side up) in the rear roller slot (paper feed)...
As shown on FR2650http://www.ebay.com/itm/Casio-FR2650TM-Printing-Calculator-FR2650TM-/122298781152?hash=item1c79934de0:g:bwEAAOSw44BYbWjv
With the PRINT feature turned on the roll should be
advancing while holding down the TOTAL key...
Hold the key until the edge of the paper roll emerges from the machine.
Hope that helps.
(now you only have to freshen up the ink).
https://en.wikipedia.org/wiki/Carnac_the_Magnificent%E2%80%8B