Replace the round brackets with square ones [] and you are good to go...Oh and btw it would be a good idea to initialize your arrays.

i have tried to change it

int i;
double D_Nath[11],D[11],er[11];
for( i=0;i < 11; i++)
{
D_Nath(i)=PI*D(i)/er(i)*.76554;
}


this is what I have for the code but I am getting a C2064 error in the line with the D_Nath(I)=
any idea what I am doing wrong

D_Nath[i]=PI*D[i]/er[i]*.76554;

You forgot to change the () for [] inside statement inside the loop