cant select any more gears than 1st n 2nd ,3rd.????

Did you already put the cases back together? I hope not. I always dump the gear cluster, drum and forks into the primary half of the center case and the roll the output shaft while turning the drum and observing the transmission. Did it shift before you took it apart, or is it something that you bought without having it run? Bent shift forks will cause it to not shift. Sometimes, you can get a fork mixed up in it's placement on the shaft or where you placed the pin in the drum. I once had an old engine that I bought a shift fork from a junkyard for. I thought that it was the right part, but it was not. I had to machine the base to get it to slide all the way to engage the gear it was supposed to. You might have a freewheeling gear in facing the wrong way on it's shaft or a slider in wrong. Hope you have some detailed diagrams to look at so you can check to see what the mistake is. If you checked before assembling the two centercases and it all turned in every gear then, but not after, check you gear shift mechanism and make sure that is assembled right.