There seems to be little chance of confusion between your software and their dating service. From your facts it's unknown how long you've been using Datepad for free notepad software and how long they've been using it for a dating service, but even assuming they've been around longer since they have a registered mark and their term shows up higher than yours on browser searches, other than the marks being identical, it seems that the Sleekcraft factors favor you. See AMF, Inc. v. Sleekcraft Boats, 599 F.2d 341 (9th Cir. 1979)
The Sleekcraft factors analyzed for likelihood of confusion are:
1. strength of the mark;
2. proximity of the goods;
3. similarity of the marks;
4. evidence of actual confusion;
5. marketing channels used;
6. type of goods and the degree of care likely to be exercised by the purchaser;
7. defendant's intent in selecting the mark; and
8. likelihood of expansion of the product lines.
Their mark may be somewhat stronger (due to longer use, registration, higher search engine results), but your respective uses have no proximity, and it seems very unlikely that either of you would expand to the others' use or would confuse each other's consumers. Your consumers are probably sophisticated computer users who have to seek out your freeware on software sites with some effort, theirs are looking for dates. Yours pay nothing for your free goods, and presumably theirs pay something for their services. It seems unlikely you had any intent of capitalizing on their mark in choosing your goods' name.
It would appear that you could co-exist in the market without hurting each other, as you probably have so far, and that adding something to your site, either a disclaimer of relationship to them or a hypelink to their site to make sure any wayward consumers find them, is unnecessary. Dating Datepad hasn't threatened a lawsuit or demanded you cease and desist your use, and their letter is pretty friendly. I suggest either that you ignore it, or send them a polite response declining their suggestion and advising them that you do not think a link is necessary because no consumers are likely to be confused between your respective uses. If they sue you, you would still have the disclaimer and link options.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.