Please use pixiedust package manager to install the avro package.
pixiedust.installPackage("com.databricks:spark-avro_2.11:3.0.0")
http://datascience.ibm.com/docs/content/analyze-data/Package-Manager.html
Install it from python 1.6 kernel since pixiedust is importable in python.(Remember this is install at your spark instance level).
Once you install it , restart the kernel and then switch to R kernel and then read the avro like this:-
df1 <- read.df("episodes.avro", source = "com.databricks.spark.avro", header = "true")
head(df1)
Complete Notebook:-
https://github.com/charles2588/bluemixsparknotebooks/raw/master/R/sparkRPackageTest.ipynb
Thanks,
Charles.