The people picker will by default show the users in the forest that the SharePoint farm is installed (that is, the resource forest). According to the documentation Peoplepicker-searchadforests: stsadm property (Office SharePoint Server) – it should also display users in all two way trusted domains and forests but you can still use the stsadm command to be sure.
For one way trusted domains (or forests) you will need to do some extra stuff.
In a one way trust you will probably need to specify a user name and a password. This requires you to run the setapppassword operation first (For more details see Select users from multiple forest domains) - STSADM.exe -o setapppassword -password key. The key you specify here is used to encrypt the password you will use to establish the people picker connection. Note: run this on every WFE of your farm.
Next you need to add the forests/domains you are going to query using the people picker.
stsadm –o setproperty –pn peoplepicker-searchadforests –pv <list of forests or domains> -url <WebApp>,<LoginName>, <Password>
where <list of forests or domains> can be something like this "forest:contoso.corp.com;domain:bar.contoso.corp.com".