There is an option in the config file of the Factorization mod for it.
At about line 50 it says "generateSilverOre=true", or at least it should say that. If this option has the value "false" instead of "true", you should just have to change it to "true" for the ore to generate.
I have not tested it out, but that should pretty much be the thing you´re looking for.
Said config file is named "factorization.cfg" and is found in your "FTBBETAA\minecraft\config\" folder.
I´m a little confused though - as far as I know this option should be enabled right from the start. If you are not the admin of the server you´re playing on, probably that ore was deliberately disabled by your admin, probably because of the laughably large amounts you find of it. (People on my server started building their floor out of silver blocks...)
If that should be case - don´t fear, there are other silver and lead ores in the game and the advantages of the slag furnace over other furnaces seem pretty dim to me, so you should be able to get around without that silver/lead ore.