Hello Rado,
if you connect to the Si5325 via USB, you must have some kind of adaptor or breed board wired to the chip. This would also explain why you have a driver software at hand.
To control the chip from LabVIEW, several options come to my mind:
a) use an USB-to-SPI interface like the NI-8451 (http://sine.ni.com/nips/cds/view/p/lang/de/nid/202368) to connect to the chip. You can then send low-level commands to the chip using the NI-845x driver's API. The SPI "command" strings that you can use to make the clock multiplier chip do something should be available in the manufacturer's documentation - NI will not be able to help you with this.
b) keep the adaptor that you currently use and call the driver software that you have already installed through DLL calls, ActiveX, .NET, TCP/IP commands, ... (whatever the software offers). This means that you will not talk directly to the Si5325, but use LabVIEW only to call specific functions already implemented in the driver software. To find out which driver functions calls are necessary to reach the desired modes of operation of your chip, you will once again want to read the manufacturer's documentation, as there is no standardized driver command set which could have be implemented in LabVIEW beforehand.
c) modifying the text files that come with the driver application in LabVIEW could be another option, but I would guess that these files are only read once each time you start the manufacturer's driver application. So you would have to manually close and re-start the driver software each time you made a settings change in your LabVIEW application, if the driver software does not offer command line parameters that allow you to download a setting to the chip and then exit the driver call directly. A first attempt to see if this was possible would be to call the driver executable that you have used before (let's call it CoolDriverApp.exe for now) from the "windows command line" (Windows » Start » Run » cmd.exe) with something like "/?" or "/h" as a command line parameter [resulting in a command string like C:\Program Files\CoolDriverDirectory> CoolDriverApp /h). If you are lucky, you will see a help text explaining the valid command line parameters.
d) NI-VISA would only be an option if the USB adaptor that you are using supports the USB_INSTR or USB_RAW class and you had (see options (a) and (b)) had the chip's command set at hand. Then you could write your own VISA Instrument Driver, which you could then call from LabVIEW.
As you can see, regardless of which path you chose, you will need more information from the chip's manufacturer...
Best regards,
Sebastian
