Hi tia,
If your chromosome names are of the form "chr1, chr2, chr3, ..." etc, you can download the following file from UCSC and just load it using "File > Load..." after selecting you custom genome.
http://hgdownload.cse.ucsc.edu/golde...refGene.txt.gz
You can rename the file, but its important to keep "refGene" in the name, its how IGV knows it is this specific UCSC table format.
Jim