I haven't watched the movie (either), but I'm sure the following snippets will help.

From Associated Press:

The $10,000 contributor, 45-year-old Canadian entrepreneur and currency exchange site XE.com co-founder Steven Dengler, ended up playing not a waiter (as the reward had advertised), but introducing a viral video with James Franco, who also makes a cameo.

Another bit of detail, this time from the movie's Production Notes:

The top-tier prize-a speaking part in the film-went to Steve Dengler. He plays the host of a viral-video show seen (and heard) onscreen and feels his money was well spent.

Do you recall any of it?

This is explained by the Veronica Mars Movie Project team in an e-mail from Rob Thomas to the Kickstarter backers, dated March 23, 2014:

What happened to the $10,000 waiter's line?

Over the last week, one of the most common questions we've received is about our $10,000 Kickstarter backer, and where his "walk on" role appeared in the movie.

As you might recall from the original Kickstarter listing, our top pledge reward offered one backer the chance to appear in the movie as a waiter, speaking the words, "Your check, sir." Many of you have correctly noted in the comments (SPOILER WARNING!) no such line -- or role -- appeared in the final movie.

For those of you who missed it, we actually released the scene as an exclusive preview on Funny or Die just a few days before the movie was released.

I'd like to tell you that we ended up cutting this Funny Or Die scene because it didn't fit with the overall tone and flow of the movie, but -- confession time -- this wasn't an actual scene from the movie. (The final shot in this skit did, however, feature a handful of local backers who volunteered to come help out as extras just a few days before the movie was released.)

The truth is that the "waiter scene" I envisioned before we launched the Kickstarter never made it into the final script. In earlier drafts, the scene took place in New York, in the first ten minutes of the movie, as Veronica explained to Piz that she'd be returning to Neptune:

(image transcribed)

       4/15/13                                               8.

                         VERONICA (CONT'D)
               You know I haven't spoken to him in 
               nine years

     The waiter arrives, sets down a check.

                         WAITER / BID-A-LINE WINNER
               Your check, sir...

                         VERONICA
               C'mon. Cheer up. You're buying me 
               dinner. Odds are I'll put out.

     Piz doesn't want to smile, but a smile escapes nevertheless.

As the script went through revisions, I ended up cutting this scene to save time, and to get Veronica back to Neptune faster.

By then, I had also gotten a chance to speak with Steven Dengler, the backer who pledged for the walk-on role -- and who had the good humor to introduce himself as "10K" while visiting the set.

Once we knew that the waiter scene wouldn't make the final script, we discussed several other options for Steve's big moment. We wanted to find something that would be memorable and fit into the movie.

In the final cut, Steve appears as the enthusiastic vlogger who introduces a viral video featuring one of our unannounced cameos:

For those of you concerned that Steve didn't get the moment he was promised, rest assured that his line -- "LET'S GET WEIRD!"-- has the exact same number of words as his original line: "Your check, sir." And for those of you who haven't seen the movie yet, trust me: he nailed it.