semPaths may not be found if you have not loaded semPlot in the session. i.e.
library(semPlot)
If you have an error from the library statement, then you may be wrong that you have installed semPlot ...
You could try
ip <- data.frame(installed.packages())
ip[which(row.names(ip)=="semPlot"),c("Package","Version")]
to see if semPlot is in your installed.packages() list