Placing a world map on r plot

I am trying to place a world map on my current plots made by level plot. This plot was made as follows:? 
? ? library(raster)
??? library(ncdf4)
??? library(maps)
??? library(maptools)
??? library(rasterVis)
??? library(ggplot2)
??? library(rgdal)
??? library(sp)
??? library(gridExtra)? ??
MFplot4<-levelplot(MFMeaner3,margin=F, at=Fcutpoints4,cuts=11, 
??? pretty=TRUE,par.settings=mapTheme, main="Historical five-day maximum?? 
??? precipitation (mm/day) model mean")
The object "MFMeaner3" has the following attributes:??? class?????? : RasterLayer 
??? dimensions? : 64, 128, 8192? (nrow, ncol, ncell)
??? resolution? : 2.8125, 2.789327? (x, y)
??? extent????? : -181.4062, 178.5938, -89.25846, 89.25846? (xmin, xmax, ymin, 
??? ymax)
??? coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
??? data source : in memory
??? names?????? : layer 
??? values????? : 0.1583802, 164.2064? (min, max)
Here was my attempt to place a world map overlay on the above plot:
? ? world.outlines<-map("world", plot=FALSE) 
??? world.outlines.sp<-map2SpatialLines(world.outlines,proj4string =?? 
??? CRS("+proj=longlat"))

? ? MFplot4 + layer(sp.lines(world.outlines.sp,col="black",lwd=0.5))However, this leads to the following error:
? ? Error: Attempted to create layer with no stat.
I also tried placing a simple world map using this:
? ? MFplot4 + plot(wrld_simpl)
But I receive this error:
??? Error in UseMethod("as.layer") : 
??? no applicable method for 'as.layer' applied to an object of class "NULL"
Why would these errors occur? 

Any assistance with this would be extremely appreciated!