Towards the bottom of the VMs window, right click on the network icon (2 monitors), and click ‘Network Settings’. Under Adapter 1, click Advanced and then ‘Port Forwarding’.
Add a new record like so, and click OK.
Name
Protocol
Host IP
Host Port
Guest IP
Guest Port
SSH
TCP
127.0.0.1
2222
10.0.2.15
22
Click OK.
Open Putty and connect to 127.0.0.1 on port 2222. You will be asked to accept the VMs key fingerprint - click Yes.