You will have to use X.509 certificate class to load certificate in wodFtpDLX.NET Certificate Property.
Here is example which will show you how to load certificate from file and connect to server with certificate:
Dim cert As System.Security.Cryptography.X509Certificates.X509Certificate2
ftp1 = New WeOnlyDo.Client.FtpDLX
cert = New System.Security.Cryptography.X509Certificates.X509Certificate2
cert.Import("c:\\certificate.pfx", "certificate_password", Security.Cryptography.X509Certificates.X509KeyStorageFlags.Exportable)
ftp1.Protocol = WeOnlyDo.Client.Protocols.FTPSwithdata
ftp1.Authentication = WeOnlyDo.Client.Authentications.Certificate
ftp1.Certificate = cert
ftp1.Hostname = "your_hostname"
ftp1.Login = "your_login"
ftp1.Password = "your_password"
ftp1.Connect()