Where should I enter my foreign (Japan) pension amount?

For entering the form:

1. Open your return to federal income
2. Select 401k, pension, 1099-R
3. Select I'll type it in myself
4. Continue
5. Select Form 1099-R
6. Continue
7. Enter the name and address of the payor
8. Federal ID number, enter the dummy code  23-1234567
9. Enter gross and taxable distribution in USD, put the entire amount in both boxes regardless of actual taxable amount in this spot
10. Box 7 - Select a code - Enter distribution code 7  for normal distribution
11. Continue

Since I haven't looked into the US Japan Treaty, I am not sure if it is taxable. Please see Claiming Tax Treaty Benefits | Internal Revenue Service.

Subtract out the income, if nontaxable:
1. Miscellaneous Income, 1099-A, 1099-C, Start
2. Scroll to the bottom
3. Other reportable income, Start
4. Other taxable income?
5. Select YES
6. Description Treaty income
7. Amount, enter your negative amount
8. Continue

I don't believe you need to file Form 8833. About Form 8833, Treaty-Based Return Position Disclosure

MUST FILE 8833

The payee must file a U.S. tax return and Form 8833 if claiming the following treaty benefits:

1. A reduction or modification in the taxation of gain or loss from the disposition of a U.S. real property interest based on a treaty.
2. A change to the source of an item of income or a deduction based on a treaty.
3. A credit for a specific foreign tax for which foreign tax credit would not be allowed by the Internal Revenue Code.

The payee must also file Form 8833 if the payee receives payments or income items totaling more than $100,000 and determines the country of residence under a treaty and not under the rules for determining alien tax status