How to show sales rep firstname in analytics dataset

Hi,

I have created a Analytics dataset for opportunities. When I add the field salesrep (employee}, it gives me a numeric number which is employee id.

I however need employee's first name. So when I use {employee.firstname}, it gives me error "your formula contains a reference to an inaccessible field employee.firstname. Please change the formula exporession"

I however can see employee's name when I type em:162 (which is the employee number) in the global search.

So even if I have access to employee's firstname field, why I am getting this error?

More importantly how do I get firstname of the salesrep in my dataset for opprtunities?