1. The Question
I put the eggfile in the 'My Agents' folder and injected one, but my norns still look like before :(

2. The Solution
Most likely this happens because you have another breed installed on the same slot. You can do two three things.
1)Leave it as it is (but that's boring)
2)Manually install the breed
3)Do a Slot Conversion
Since there is a seperate slot conversion tutorial, we'll focus on the 2nd option here. How can you get rid of a breed to install another?

Step 1 (optional)
This step is only needed when you know that the creatures grow unusually. For instance, the reindeer norns grow at the youth age (stage 3) and not the adolescent (stage 2) and adult (stage 4) ages. If you don't delete the old files, you'll have a breed that uses both images and will look rather weird at times. Also, if the existing breed has an old age (Stage 5) and the new one doesn't, you'll end up with norns from one breed looking like the one you wanted to delete at old age.
You go about deleting by visiting your BodyData and Images folder and searching the right files. Search for these terms: * (genus) (age) (slot).
  * is the body part, and is usually left the wildcard sign.
  (genus): 0 and 4 for norn, 1and 5 for grendel, 2 and 6 for ettin, 3 and 7 for geat
  (age): 0 (baby), 1 (child), 2 (adolescent), 3 (youth), 4 (adult), 5 (old), 6 (senile)
  (slot): the slot your creature is in.

Step 2
Put the Eggfile that contains the breed you want to install in a seperate, empty folder (this isn't crucial, but it will make things much easier). Download Revelation, or a tool that works in a similar fashion (extracting the files inside the agent). Double Click on your agent file after you've installed it, and you'll notice all files appearing in your folder.

Step 3
Now it's up to you to replace the old files with the new.
spr, s16 and c16 files go into your 'Images' directory
att files go into your Body Data directory.

Step 4
Start Creatures and hatch an egg. Normally things should work now. If they don't, feel free to contact me and tell me what the problem is.