OptiRex - 12:54 am on Oct 18, 2006 (gmt 0)

to ask visitors to click on those product referrals, download, and install them.

Just a cursory look through the T&Cs brings up this:

Incentives Web pages may not include incentives of any kind for users to click on ads. This includes encouraging users to click on the ads or to visit the advertisers' sites as well as drawing any undue attention to the ads. For example, your site cannot contain phrases such as "click here," "support us," "visit these links," or other similar language that could apply to any ad, regardless of content. These activities are strictly prohibited in order to avoid potential inflation of advertiser costs. In addition, publishers may not bring unnatural attention to sites displaying ads or referral buttons through unsolicited mass emails or unwanted advertisements on third-party websites. Publishers are also not permitted to use deceptive or unnatural means to draw attention to or incite clicks on referral buttons.

The above specifically refers to "users to click on ads" and "use deceptive or unnatural means to draw attention to or incite clicks on referral buttons."

Vague, very vague, I would ask, it does not specifically say NOT to ask visitors to click on referral buttons and asking visitors directly would not be deceptive nor unnatural, would it?

I doubt they want you to do that however there's no harm in asking.