Custom Content Installation issue.

Had this come up last night while trying to get some things installed in the game via the game launcher... any ideas on how to fix it?

Try clicking it away; if it shows up persistently, then this older topic here might give some hints:
https://www.games4theworld.club/t16882-the-sims-3-launcher-problems-notice

The_gh0stm4n wrote:3) Sometimes, when opening up the launcher, you'll get a window saying something like "an error has occurred on the script of this page". If you get that, click on "Yes", and in *most* cases, that window shouldn't pop up again. If it still persists, you could try updating your Adobe Flash player and Java too (these update notifications should come up automatically on your taskbar, as soon as they're available).

As a very radical measure, if this window pops up again and again, try resetting your Internet Explorer. In Windows Vista/7/8, this can be done by going to the System Control Panel, then Network and Internet -> Internet Options -> Advanced (tab) -> Reset Internet Explorer Settings (Reset). After the reset, try the launcher again.

You can - and actually should - prefer using CC Magic when installing .sims3pack custom content. The tool can install all types of .sims3pack items, except worlds.

Worlds still need to be installed with the launcher.