could not find function "geneSim"

You likely made a typo: the name of the library is GOSemSim (and not GoSemSim)!

> library(GOSemSim)
> d <- godata('org.Hs.eg.db', ont="MF", computeIC=FALSE)
preparing gene to GO mapping data...
> geneSim("241", "251", semData=d, measure="Wang")
$geneSim
[1] 0.149

$GO1
[1] "GO:0004364" "GO:0004464" "GO:0004602" "GO:0005515" "GO:0047485"
[6] "GO:0050544"

$GO2
[1] "GO:0004035"

>