Man 4e is a lot more complicated than 2nd Edition.
Why cant I be a Daggermaster and also do the Martial Archetype epic destiny thing ?
I have read it and thought I had understood it ?
If I go Paragon path in something, does that mean that I cant do the Epic Destiny stuff ?
Well, it helps if you think about it this way: At 1st level (the start of the Heroic tier), you pick a class. At 11th level (the start of the Paragon Tier) you pick a paragon path. At 21st level (the start of the Epic tier) you pick an epic destiny.
Your choice of class will affect which paragon paths become available to you, and your choice of paragon path may similarly affect which epic destinies are available to you.
For example, Daggermaster has the prerequisite of Rogue class. This means that in order to select the Daggermaster paragon path at 11th level, you must have selected the rogue class at 1st level, or multiclassed into Rogue by taking a rogue multiclass feat such as Sneak of Shadows.
Martial Archetype has the prerequisites of any martial class and paragon multiclassing in a martial class. Paragon multiclassing means that you take another base class as your paragon path, and you gain powers from that class instead of powers from another paragon path. For example, if you are a Rogue, you must paragon multiclass into another martial class, e.g. Fighter, Ranger or Warlord, instead of taking the Daggermaster paragon path in order to take Martial Archetype at 21st level.
I recall from another thread that you are playing in a relatively low-level game at the moment. I think one factor that is increasing the confusion is that you are looking too far ahead. In my view, 4e is quite easy to understand if you take it one step at a time.