"Why can't I find human VH3-23 sequences?"
Unfortunately, there are many different gene naming formats, and iReceptor can't search over all of them. We have chosen to use the widely accepted IMGT Nomenclature. This means you can search for "IGHV3-23" under the V-Gene Allele filter on the Sequence Search page. This search does have to begin with "IGH", but you can widen your results by including less of the gene, e.g. by writing only "IGHV3".
"What's the difference between Bookmark this search and Download these sequences?"
Bookmarking a search is a in-browser action that just saves the parameters of the search that you did. Downloading sequences saves the actual results of the parameters of your search to your computer's hard drive.