The following advice assumes that you have a significant amount already in the account in cash equivalents. If you are only talking a few hundred bucks or so, then just jump in at the next dip (like today's).
If you have a larger amount to move into equities, the safest approach is to gradually move it into investments over some period of time at regular intervals regardless of what is going on in the market. This mitigates the risk of investing it all into an fund that is peaking at the exact moment you buy.
So, for example, you might invest 20% of the total amount each month for 5 months to gradually get into the market. The larger the amount you are investing, the more you probably want to spread it out, but don't spread it out much further than a year or you are losing opportunity cost by leaving your money in cash-type investments with likely a very poor rate of return.
This strategy is called dollar-cost averaging if you want to research it more.