I've participated in an ICO by sending Ether from my Jaxx wallet, how can I make the new token show in myetherwallet?
You need the token contract address and number of decimals.
The company doing the ICO should provide you with this info. If not, you can search for the token on Etherscan. You can also find it by inspecting your transaction on Etherscan.
Here is a picture that shows you what to do once you have gotten hold of the contract address and amount of decimals (just choose Main Network instead of Kovan):