This is usually caused when the author puts files in the archive that are not named exactly as they are referenced in the library file. Read the readme.txt file to see if there are any conversion instructions. Sometimes (particularly with bump maps) authors upload files that are in a different format because they take up less space. You then have to convert them. Some times the author just goofs & includes the wrong files which are usually files that were used in the character's developement. Also, make sure you followed all the instructions for file placement. If the file is not where Poser thinks it should be you will have to point to it.