Soulver doesn't let you do this with an operation, but there are a few ways to do it. First of all, there's the old fashioned way:
Add your numbers up (in parentheses) and then divide by the number of numbers.
There's also a more automated but less convenient way. Soulver can calculate the average of the results (blue column) that are currently in your document:
But, if you want to use that average later, you'll have to do a bit more.
Select Average, then click on
Copy Selected Statistic.
Add a new line to your Soulver document:
avg = and then paste in the average. You now have an average that you can rever to via