You use the Ranks in the Primary Power (Object Mimicary) to determine a secondary Points pool that you can use to purchase other powers.
Device, Alternate Form, Battle Form, and a host of other M&M powers actually give you more points than you would normally be able to use by placing limitations on your ability to use the powers.
I don't have my book on me so let me use a power I know better as an example: Device.
Device has two options: 3 points a rank or 4 points a rank.
At 3 points the object is something you hold and that can be disarmed to get it away from you. At 4 points it's something attached to you that someone has to disable you to get away. In both cases, your power comes from an external device which can be taken away.
For each rank you buy in Device, you get 5 new points which form a secondary pool of points. You use these points to buy the Powers the Device gives you.
Object Mimicry works the same way (I think). You use your secondary pool to purchase powers which are in keeping with the theme of whatever sustance you mimic.
So, if your mimicing stone or metal than you use those points to spread between Density, Immunity, and Protection. If you buy 10 Ranks in Object Mimicry and each rank gives you 5 points than you have a pool of 50 points.
Density is 4 points a rank, I think. Protection is 1. Immunity is also 1.
So, let's start with Density. We'll assume you don't turn into rock completely, just get a rocky coating so Density 5 for 20 points which causes you to gain several other powers by default. Next your Protection. 10 is the normal max for a PL10 character without a trade off and we'll assume you don't want to trade so 10 points in for 10 ranks. You've got the rocky coating so your basically bullet proofed, we'll back that base 10 with 5 Impervius, making normal guns (up to normal rifle fire) totally ineffective. So, 15 points on Protection makes 35 out of 50, giving us 15 points for Immunities. Inside the rock shell you don't want to have to worry about air so 9 goes to Life Support, right off the bat. With the other 6 we'll get some limited Immunities. I think for 3 we can select a narrow type so we'll go for bladed weapons and fire. I'll have to double check the Immunity numbers but you get the general idea.