You need to do two things to set up what you want. You need to set up the port forwarding and then also allow the traffic through the firewall.
In ASDM, go to Configuration -> Firewall.
To allow traffic through the firewall, go to Access Rules and click Add, then enter these values:
Interface: outside
Action: Permit
Source: any
Destination: 1.1.1.1 (your external IP address)
Server: tcp/3389
To add the port forwarding rule, go to NAT Rules (below Access Rules) and click Add, then Add Static NAT Rule..., then put these in:
Original
Interface: inside
Source: 10.10.10.10
Translated
Interface: outside
Use IP Address: 1.1.1.1
Port Address Translation (PAT)
Enable Port Address Translation (PAT)
Protocol: TCP
Original Port: 3389
Translated Port: 3389