In your list of scene elements, right click on Objects and choose Expand All. This will open up all the keyframed bones and their actual keyframes. You screen should look like this.

The scene list has all your bone names, and to the right in your time frame, you'll see little dots. These dots are your keyframes for all your bones. We'll need to copy 2 more sets of keys to fit into your timeline. Drag a selection box around all the keyframes. Now, while holding down your select key, drag your keyframes to the right, so the far left row of the new keys match up with the far right row of the original keys. Do this again so the timeline is completely filled with keys as illustrated below.

Once your keys look like this, you may close this window to return to max.

Again, go into your Time Config and set your starting frame to 24 and your ending frame to 48. Now, once you play back your animation, as long as you've done everything properly, the pop in your loop is gone. Save this motion to a .bip file for future use. If you need a walk cycle in the future, this is the perfect file to work off of. Your character should now be walking perfectly, but without personality. From here, the rest is up to you. This is a base walk cycle that you can build on by adding or adjusting existing keyframes and timing until you get the results you're looking for. Add in some more spine motion, give him a head bob, swing the feet out, or put more sway into your hips… Just have fun, or wait until I finish the tutorial on "How to Add Personality to a Generic Walk Cycle". Hope this helps!