when you import the texture in there should be a list of formats next to the preview, make sure that it is in CI8 format, and its palette is in RGB5A3 format, like Msng said.
Now, here is brawlbox (with fitluigi00 open): If we click the small '+' symbol next to where it says model data, then again on the one next to the folder. we should get this: Now if we right click and select preview we get this: We can't see anything yet, so scroll down to zoom out and the right click and drag to pan (you can also use the direction keys. To rotate, hold Ctrl then pan normally) now we should be able to see the model If this doesn't work, You computer might not support the software otherwise I don't know what you are doing wrong
once on preview, scroll down to zoom out and use right click to pan. If you still can't see it, you will need to download the windows update which you can get at the bottom of the brawlvault resource section
I presume that means 'what next' If you want you can check out what it looks like in the model data tab, by selecting the model and clicking preview (If this doesn't work, there is a download to fix it somewhere around here, but I don't know where) Then if you are happy with what you have, then go to file / save as and save it as .pcs (and .pac, just to be sure)
And finally put the .pcs and .pac into Private/wii/app/rsbe/pf/fighter/(name of fighter) in your SD card Then its ready to be used in Brawl
If it's just texture editting, Open the pac file in Brawlbox and expand the texture data tab (normally the highest one relates to the character) and export any textures you want to edit. Open the exported textures in an editting program (photoshop, paint etc.) and edit away. when your done save it then return to the texture tab in brawlbox. Select the same texture you exported and right click, go to replace and select the editted file then 'Okay' it. Voala, editted texture!
There are alot of things you can do in brawlbox, but I am not educated in all of them. You should check out this guys video tutorials, they are very useful http://www.youtube.com/user/WackaAlpaca