writing information into avalon slave in c

From NIOS you just treat it as if it is any other bit of memory and the processor will take care of the signalling (hence they are called memory-mapped).

There are handy functions provided in the NIOS BSP that are useful when writing to slave devices such as:

IOWR_32DIRECT(<base address>,<offset in bytes>,<32bit data>);

IORD_32DIRECT(<base address>,<offset in bytes>);

And similar ones for 8bit and 16 bit data.

Alternative you can do something like:

int main(){

char* x = 0x3008;

*x = 0x0F;

}