i happened to purchase imax B6 charger and using 1.2 V NIMH batteries 2500 MAH as batteries for my plane receiver.............. dont know with what volt current i should charge and discharge it and how it is calculated can somebody please guide me on this

you can very well charge at 2.5 A recommended howevr NiMHs can be charged upto 3C that is this cell 6A max but not recommended depends on how well the battery can respond. All the best.

In Ni-Cd and Ni-MH batteries the charging current rate depends on the size of the battery cells and type of the cells. whether they can withstand fast charge/discharge cycles. If you are using AA size cells they are not suitable for high charge discharge rates. For them use the  charge currents recommended by their manufacturers and shown on the cell. Never charge AA cells at more than 1 amp.
Larger sized cells like A, 2/3 A, Sub-C can be charged at higher currant rates because they are designed for High charge discharge currents. I charge my Ni-MH batterypacks used to power flight motors at 1C level even if they can be charged at 2C or 3C levels. I believe this increases the life of these batterypacks to the maximum possible level.

Also heat is the no-1 enemy of Ni-xx batteries. They should be Luke warm when fully charged. Constantly monitor the battery temp by touching them and feeling the temperature every few seconds and if temp increases rapidly terminate the charging and remove the battery from the charger and let it cool. Ni-mh battery become slightly warm when charging and it is completely normal. But if you keep on charging even after the battery is fully charged the temperature of the batteries increases very rapidly and destroys the battery pack.

Regards,
Rao. 

Am in total agreement. Try and use the LOWEST possible rate, unless you are in a hurry. I, almost invariably, use trickle charging. Completely safe even if left on indefinitely.