Well, the sim object is of class simper and it does not make much sense to write it as CSV rectangular data.
If you are using RStudio, then in the Environment pane you will see that the 'Value' column is 'List of 6' and you should be able to view the object as a list.
If you just want a text representation of your object which you can retrieve later, you can dput and dget. See code below.
library("vegan")
data(dune.env)
data(dune)
(sim=with(dune.env, simper(dune, Management)))
summary(sim)
class(sim)
dput(sim, file = "sim.txt")
sim2 <- dget("sim.txt")
summary(sim2)