It sounds like you have tried setting up the SSH option via a Configurable Option, and also as an Addon. This should probably be a Configurable Option only, and not an Addon.
If I understand right, the checkbox would toggle itself during checkout as a Configurable Option. This doesn't sound right, but it does make me wonder if the option is fully configured.
Edit the Configurable Option under Package > Options > Options link. Within the Options section, there should be a Name and Value. Both of these must be configured, and a pricing term must be set, even if the price is 0.00. A price term must be set for your 1, 3, 6 month and 1 year terms in order for the option to be visible for each term. If you only want the SSH checkbox to appear for 1 year terms, you can just enter a 1 Year price for the option, for example.
If that doesn't work, maybe you can post (or email me sales@) a screenshot of your configurable option settings for this item.