It is purely a string:
startInfo.Arguments = "-sk server -sky exchange -pe -n CN=localhost -ir LocalMachine -is Root -ic MyCA.cer -sr LocalMachine -ss My MyAdHocTestCert.cer"
Of course, when arguments contain whitespaces you'll have to escape them using \" \", like:
"... -ss \"My MyAdHocTestCert.cer\""
See MSDN for this.