The n-th Ternary

CJam, 18 18 bytes

ri,:)":?0"*2/ze_S*

Try it online.

Explanation

ri,:)          e# Generate the list 1..n.
":?0"*         e# Insert ":?0" between every two numbers.
2/             e# Split into pairs, e.g. 1:, ?0, 2:, ?0, ..., ?0, n.
z              e# First items in every pair before second items in every pair.
e_             e# Concatenate the two parts.
S*             e# Insert spaces.

Ruby, 31 bytes

f=->n{[*1..n]*' ? '+' : 0'*~-n}

Test:

> f[1]
=> "1"
> f[7]
=> "1 ? 2 ? 3 ? 4 ? 5 ? 6 ? 7 : 0 : 0 : 0 : 0 : 0 : 0"

Pyth - 19 18 17 bytes

The spaces are killing me, thinking of a better way to handle them.

+j" ? "SQ*tQ" : 0

It just joins the numbers by a " ? " and then concatenates the second part on.

+              String concatenation
 j" ? "        Join by the string
  SQ           1-indexed inclusive range to input
 *             String repetition
  tQ           Input - 1
  " : 0        String implicitly closed by end of program

Try it online here.