The 5 just means five steps above the root. Like you said a D5, count up five notes D, E, F, G, A. So the fifth above D is A. Placed together as a chord it would be D5. The chord would look like this.

E:-----
B:-----
G:--7-- <- octave of root
D:--7-- <- the fifth A
A:--5-- <- the root D
E:-----