If the package is not installed, then get the package from bioconductor:
{{{
source("http://bioconductor.org/biocLite.R")
biocLite("yeast2.db")
}}}

load the library and list the contents of the package:
{{{
library(yeast2.db)
> ls("package:yeast2.db")
 [1] "yeast2"              "yeast2_dbconn"       "yeast2_dbfile"      
 [4] "yeast2_dbInfo"       "yeast2_dbschema"     "yeast2ALIAS"        
 [7] "yeast2ALIAS2PROBE"   "yeast2CHR"           "yeast2CHRLENGTHS"   
[10] "yeast2CHRLOC"        "yeast2CHRLOCEND"     "yeast2DESCRIPTION"  
[13] "yeast2ENSEMBL"       "yeast2ENSEMBL2PROBE" "yeast2ENZYME"       
[16] "yeast2ENZYME2PROBE"  "yeast2GENENAME"      "yeast2GO"           
[19] "yeast2GO2ALLPROBES"  "yeast2GO2PROBE"      "yeast2MAPCOUNTS"    
[22] "yeast2ORF"           "yeast2ORGANISM"      "yeast2ORGPKG"       
[25] "yeast2PATH"          "yeast2PATH2PROBE"    "yeast2PMID"         
[28] "yeast2PMID2PROBE"
}}}