It often helps us analyze your situation, and work out how to help, if you supply a packaged workbook. There are some great guidelines published here. Doing this will often get you a faster reply.
Easiest way to do is to select analysis > view data and in the dialog containing the data select all, the copy and paste it back into Tableau. It will get the data and cluster assignments into a new data source. Since it is a new table, you can use them as dimensions if you like.
Any reason not to use Tableau's built-in k-means clustering? Are you using a different clustering algorithm?