A .1uF capacitor has a capacitive reactance of 13 ohms at 120KHz. The 18uH inductor cancels that out, providing a low-impedance path for the 120KHz signal. I use .22uF and 8.2uH in the XTB for the same function.
I agree with Brian that capacitors rated for 250VAC across line operation are the best for this application. However, a metalized polyester capacitor rated for 600 or 630VDC should also be safe. The ones I use from CDE are rated 630VDC / 250VAC.
FYI: Unless they changed it recently, the capacitor in a 4816H SignaLink is only rated 400VDC.
Brian is also correct that a capacitor should not be installed across two breakers in the distribution panel as it will violate the electrical code.
The point I was trying to make earlier is that the SignaLink is a very inexpensive coupler in a convenient package. The XPCP made by X10 is a much more elaborate unit with twin tuned circuits and complete isolation between them. That coupler is code compliant when installed in a standard electrical box adjacent to the utility panel.
Jeff