Hi,
In order to use R Visuals you need to first install R and enable it in Power BI Desktop
Follow these steps to use R Visuals in Query Editor:
Step 1: 
•  Load your data into Power BI Desktop
•  Get Data > CSV from the Home ribbon in Power BI Desktop
Step 2: 
•  Select the file and Select Open
•  CSV file is displayed in the CSV file dialog
Step 3: 
Once the data is loaded you will be able to see it in the Fields pane in Power BI Desktop
Step 4:
Open Query Editor  by selecting Edit Queries from the Home tab in Power BI Desktop
Step 5: 
In Transform tab, select RUN R Script and the Run R Script editor appears
Step 6: 
Enter your R Script in the R Script Editor      
   Note: You will need mice library installed in you R  environment for your script code to work properly
   library(mice)
   tempData <- mice(dataset,m=1,maxit=50,meth='pmm',seed=100)
   completedData <- complete(tempData,1)
   output <- dataset
   output$completedValues <- completedData$"SMI missing values"
Step 7: 
After selecting OK, Query Editor displays a warning about data privacy
Step 8: 
For R Scripts to run properly, all the data sources must be public
Now you can finally create your own visualizations depending upon your dataset.
Hope This Helps!!