Minecraft runs very specifically especially when installing mods and the smallest error as you know can cause the mod to crash! >:( but do not fret! I am here to help!
Let's start. Here I have gathered a bunch of crashes around the Pixelmon forum and compiled them into one universal tutorial anyone can understand!
The following messages are found under the section that says description, on the top of the report.
"Java Version: 1.6.0_65"
If this shows up in your system details of Minecraft crash report you are running the wrong Java. As of 3/12/14 it is 1.7 (Java version, not Minecraft, in case of confusion) so please update your Java.
"java.lang.IllegalStateException: cannot determine the type of the type parameter 'REQ': class "
or
"java.lang.NoClassDefFoundError: net/minecraftforge/common/util/EnumHelper"
or
"java.lang.NoSuchMethodError: net.minecraft.client.gui.ScaledResolution.<init>"
Make sure you are running the right 1.7.2 version of Forge. You can find the proper version of Forge needed for Pixelmon on the Pixelmon download page.
If your crash report doesn't match any of these issues, post the full report in the Support Tracker.
I will add more as I find more posts on the forums ^_^ happy crashing. I think...