Instaed of "uses DEtools" you should say "with(Detools)". For example, try the following:

>restart:

>solDE:=proc(DE,function,variable,IC,lowrange,uprange)

          local x,a,b;

          x:=variable:

          a:=lowrange:b:=uprange:

          with(DEtools):

          DEplot(DE,function,x=a..b,[IC]);

          end proc:

>DE:=diff(y(x),x)=5*y(x);

>IC:=y(0)=10;

>solDE(DE,y(x),x,IC,-5,5);

with kind regards

F. F.