In your R console or RStudio, you can invoke .libPaths() to get a list of where R will check for packages.
.libPaths()
[1] "/home/grosscol/R/x86_64-pc-linux-gnu-library/dev" "/usr/local/lib/R/site-library"
[3] "/usr/lib/R/site-library" "/usr/lib/R/library"
However, I don't think that's the problem looking more closely at it. @rpath isn't referring to R's library paths. That's an error being thrown from .so file which is a compiled library. It's looking for another library at runtime.
Found a neat description of that here:
@rpath stands for Run path search path. So it's a search path for the linker. Runtime Search Path instructs the dynamic linker to search a list of paths in order, to locate the dynamic library.
When you installed the package, was it built from source? If so, I think prophet.so is not finding libtbb.dylib in the locations it was assumed it would be in. Does libttb.dylib exist anywhere on your system?