for a process you need either a sensitivity list or a wait statement within. a (not synthesisable but simulatable) version of your process could look as follows:
process
variable count: integer;
begin
count := 0;
while (count < 32) loop
wait until rising_edge(clk);-- if rising_edge(clk) then
twobits(0) <= mpcnd(count);
twobits(1) <= mpcnd(count+1);
count := count + 2;
--end if;
end loop;
end process;