To setup DKIM signing in Mimecast you need to create a definition and a policy
Setting up a definition
Login to Mimecast
Select Administration console
From the top left select Administration > Gateway > policies
Select definitions drop down select DNS Authentication - Outbound
Select New DNS Authentication - Outbound signing
Set a name for the definition
Tick the checkbox Sign outbound mail with DKIM
Use the lookup option to select domain to DKIM sign
Select generate
Create a TXT record within your DNS with the public key provided save and exit
Verify public key in Mimecast console
Setting up a policy
Login to Mimecast
Select Administration Console
From the top left select Administration > Gateway > policies
Search for DNS authentication outbound
Select New Policy
Name the policy
Under emails from use address based on "Both"
Save and Exit
Create a free OnDMARC account to test your configuration.